I’m… on the opposite end of the spectrum, eheh. I see quite a few people on the forums saying they’re 30, 40, and older, but there’s not too many under 16 and I’m one of them.
I really don’t think language learning gets much harder once you start gaining a sense of awareness. In fact, it probably gets easier because some concepts are explained in a complicated manner (at least in terms of grammar).
Between the ages of 0 to 7 is when things seem to be the best, and obviously any WaniKani user would be passed that point. 8 to 12 is an awkward period in that the person isn’t really too aware of what they’re doing… they’re older than the supposed “language learning critical period”, but too young to understand grammar points or anything that would be taught to older students. It’s not impossible to learn a language during this period, but it’d probably require an immersive classroom environment.
For example, I grew up speaking some Cantonese but stopped being in a Cantonese environment after about age 3 or 4. Nowadays, I can’t say much other than “I need to go to the washroom” and “I want to eat” and incredibly basic things… can understand a tad more than I can speak though. From about age 5 to 9, I was in a Chinese school teaching Mandarin (about two hours per class, once a week), but my Mandarin sentence construction ability goes about as far as “I am (x)” (and this is something I ended up learning while messing with language learning apps a couple years ago…) - the only thing useful I’ve retained from then is being able to read pinyin. From ages 10 to the present, I ended up in a French immersion program. The first year of this program, all instruction was in French - in fact, English was pretty much only used during break/lunch. That was probably the year I learned the most French, and it certainly paid off because French is my most strong foreign language as of yet.
That’s just my take on things, though - take it as you will. Perseverance is key, and it applies to anyone regardless of age.