Considering your freeweb pictures and stats, I'd say you have absolutely NO muscle to lose. You can only go up from here, assuming you ever do.
Listen, kid...stop wasting time with these worthless questions you keep posting on this site, pick a goal (gain muscle = gaining weight, lose fat =losing some weight, since you can;t build muscle and lose fat for too long, unless you're gifted).
I'm sorry if I come across as too harsh, but I'm (probably) thrice your age, and do know youre wasting precious youth making up excuses for a lack of progress.
Stay strong, youngblood.
And about your original question, progressive overload and periodization will keep you gaining muscle well into your sixties from what I've seen.