The optimal age to gain native-speaker fluency is considered to be 0 - 10. The brain’s plasticity starts to wain from there. But it, barring some physical conditions, does remain plastic so learning continues throughout life.
@Kellii, you don’t have to wait for retirement. Spending 15 minutes a day on an app like Duolingo (there’s a free version) can be a ton of fun! Heck, you can even learn Klingon there 
We started in third grade (as ca. nine-year-olds). Today, the kids start learning English in first grade (and they start school one year earlier than I did, as six-year-olds). In eighth grade, they have to choose another language elective, typically French or German, but some schools also offer Spanish or even other languages, depending on the languages the teachers are proficient in.
I have thought about it. I could always do that plus the community college when I have more time on my hands I suppose. I do occasionally pull up a Spanish language show and try to figure out what they’re up to. I probably get it comically wrong most of the time 
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I did very much enjoy Spanish class. I think some of the bigger schools here now start students younger. I don’t hear Spanish around here as much as I used to, I’d never heard it as a child but it had become pretty prevalent around here before Trump’s first term. He has definitely changed the atmosphere for the worse. LA is looking like a war zone. He’s deploying the military against the protesters saying “When the looting starts, the looting starts”. I hadn’t heard of any looting. I think he’s enjoying this.
I have a slightly different take on this.
In the US, we can travel thousands of miles and the language doesn’t change. Oh yeah, there are accents and sometimes they are hard to understand, but at least there is something familiar on which to base understanding.
There aren’t many places in this world like that - and I think necessity is the mother of invention, so learning a language you don’t get to practice and don’t need pretty much stifles all impetus to learn a different one.
Latin and classical Greek were useful in the past, because of their role as official or semi-official languages of discourse within the Catholic Church, which, of course wielded serious political power in Europe up to around the end of the 18th century. They became, in combination, the de facto means of international communication in Europe and official languages of academic discourse as well - hence their choice as the base languages for the Linnaean system of classifiction.
But from a more practical standpoint, they also possessed utility value, because both languages are based upon a systematic grammar, involving complete noun declension and verb conjugation (features that English has increasingly discarded). Thus, English speakers, upon encountering these languages, are first introduced to linguistic diversity, thence to a much deeper understanding of syntax and grammar, which improves understanding of English itself.
Along the way, an added bonus arises due to the manner in which English borrowed a fair chunk of its vocabulary from the two languages, either directly, or by proxy through French, with of course the fairly significant German influence arising from the Saxons.
But, one key aspect of Latin and Classical Greek, arising from the presence of complete declension, is that word order in these languages is far less important. Because the exact grammatical function of words in these languages is denoted by spelling changes (particularly changes in word endings), they don’t need to be placed in a particular order in a sentence, in order for meaning to be conveyed accurately. Word order can therefore be changed in order to provide emphasis, without destroying meaning.
This, combined with the brevity arising from complete declension, forces one to think carefully about sentence construction in both languages. This has a side effect when considering English sentence construction, where word order is more important in order to convey meaning properly, due to the increasing discarding of declension.
Once the requisite good habits are developed, they tend to persist, and this results in those whose education included Latin and Classical Greek, exhibiting a particular language style, both in speech and writing. Both an extended vocabulary and careful sentence construction, tend to be hallmarks of such an education, as well as a tendency to think first and speak or write later.
As an example of the nuances that arise, I’ll provide an example from Greek. While most Indo-European languages have two voices for verbs, namely active and passive, Greek has three - active, middle and passive. To make matters worse for elementary students, the middle and passive forms are identical in spelling, and have to be inferred from context. But that’s a complication to be covered some other time.
What matters here is that for certain verbs, the active form is used to denote an action pursued on the basis of seeking to apply high principle, while the middle form is use to denote an action pursued on the basis of personal gain. The prime example being the two forms δικαζω and δικαζομαι, of the verb that translates from Greek as “to seek legal redress”. The first form is the active form, and is used when referring to the actions in court by the judges, who, in theory at least, are seeking to apply the law from the standpoint of high principle. The second form is the middle form, used when referring to the actions in court of the litigants, who are seeking to gain personally from said actions.
I won’t bother with the full conjugations, because they are lengthy, and in places tedious, and involve in addition extra features of Greek not seen in other languages, such as the existence of the optative mood, and the preference for aspect over tense. Already, even the novice reader should be aware that there is a lot to learn here.
The use of the spelling for both middle and passive forms, appears to be a matter of convenience, relying upon the fact that very few verbs require both a middle and passive form, and one learns which verbs involve middle or passive usage. But it’s a feature that causes consternation among students during the early stages of learning Classical Greek.
All of this, of course, before one learns about various dialects (which have their own spelling changes, just to annoy the already hard-pressed student), such as Doric, Ionic, Cypriot, and the special dialects used in various literary genres - epic, lyric and Homeric, or the conventions that apply in various strands of Greek drama and theatre - Greek comedy, for example, introduces its own idioms for good measure. Then we throw in to the mix Koiné Greek (the lingua franca of Mediterranean trade), which was used for many New Testament writings. That term originates from Greek itself - η κοινε διαλεκτος, “the common speech”.
Those who persist to the level of becoming actual Greek scholars, and who master this baroque edifice, tend to have their own speech idioms in English as a result. Which, of course, should not surprise the astute.
What made deciphering many Greek texts from before the fifth century BCE such a headache, was that said texts were written entirely in capital letters, with NO gaps between words, NO accents providing pronunciation clues, and NO punctuation. Greek at that time was the ultimate “wall of text” language.
Therefore, a literate Greek citizen of the time, required to apply a high level of concentration, when reading texts in his or her own language, especially since word endings were critical for correct determination of meaning. ALL of the features seen in teaching materials - the use of small letters, accents, punctuation, etc, were features added later.
Latin at least had the benefit of using the same alphabet as English, a near identical set of grammatical constructs, and having, like English, a tense based system for verbs. But it has its own idiosyncrasies, including nouns of incomplete declension and the occasional verb featuring incomplete conjugation, which have to be rote learned because there’s no other way. Along with similarly arcane rules to Greek, with respect to which noun cases are associated with specific prepositions.
Immerse a student in all of this for five years, and you need to be sure that your instruction is top class. Otherwise, you risk shell-shocking the student into being utterly repelled by linguistics for good. Though sometimes you find students who enjoy the, well game like nature of it all, and who tend to become skilled bureaucrats in later life. Not for nothing was the British Civil Service fond of rococo bureaucracy during the years of “classical education”, particularly when we still had an Empire.
One day, enough people on the Internet might decide that this is a whole new arena of mischievous fun, and revive the hilarity that would otherwise be in danger of dying out. Beware the Ides of March if they do.