Just to cheer you up a bit, I taught EFL (English as a Foreign Language) for 5 years and did quite a lot of study into language acquisition. I now firmly believe that it would not have been any easier for you to learn a language at 16 than at your current age. In fact, you have many other advantages at an older age that would argue that you will do much better now.

I could write a book on the subject, but very quickly, the myth that children learn faster than adults is just a myth. In fact, the average child (who is doing almost nothing other than studying their native language in every waking moment) learns about 1000 new word families per year up until they reach 20 (at which point they will have acquired about 20,000 word families). "Word family" means a word and it's associated extensions -- police and policeman are one word family. It also includes conjugation/inflection of verbs, etc.

The average 5 year old has about 5000 word families and probably about 2000 grammatical constructs. It takes them 5 years to learn that. Talk to a 5 year old so you can get a sense of what kind of proficiency that means ;-). A 10 year old has about 10,000 word families and about 3000 grammatical structures (basically all except very strange ones). They can't read a newspaper and understand it, but they can watch your average movie or TV show without any difficulty. To achieve adult level proficiency with an ability to read news papers, read technical documentation, write well in your job, etc. takes about 18-20 years. Many, many people in the general population fail to reach that level.

Your failure has very little to do with your age. If you spent 10 years studying French full time, you could do much better than a 10 year old. Your failure has to do with the fact that languages are taught very, very, very poorly at schools (even private night schools). There are unreasonable expectations (like 2000 words of vocabulary -- words, not word families! -- will make you "fluent"). Also, the instruction tends to focus on teaching you grammatical rules from which you tortuously try to construct sentences. If you diligently apply yourself you will be able to construct many grammatically correct sentences... which are idiomatically wrong and hence unhelpful unless you have very accommodating conversation partners.

My big helpful hints:

- Don't compare your fluency/proficiency in French with that of your native language. It will take you at least 20 years to reach that level, unless you find some amazing shortcuts.

- Learn languages by sentence, rather than by vocabulary or grammar. Take an exemplar that you don't know the meaning to and learn it. Don't bother with grammar rules unless you just happen to like grammar.

- If you happen to like grammar, study it in French, not English. This will give you a wonderful supply of exemplar sentences to learn.

- Study frequently for short periods of time. 15 minutes 3 times a day will give you much more ability than a 10 hour session once a week.

- Study realistic sentences. Read books. Doesn't matter what. Just struggle through and memorize the meaning of any exemplars that you don't know.

- Watch TV with subtitles. As much as you can.

- Get a lot of French music (with lyrics). Learn them and sing them with the original. Record yourself and study the differences.

- Don't ever give up. Ever. Even if you give up, start again. Start today. Start now. It's only 15 minutes. How is 15 minutes going to hurt you?

- Find a reason to continue doing this. It doesn't matter what it is, but you need reasons to fuel you for at least the next 5-10 years.

Your 16 year old self may have been able to comprehend the basic of basic french basics as they shoved it down your throat at school (possibly without actually letting you speak). Your older self actually has a chance to learn the language with fluency and proficiency as long as you take a realistic approach. Good luck!