What? why the hell would you be too old? Some older guy, mighta been around 36 commented to me how he wished he could ride a bike like us youngsters do. I explained to him that I only just got back into it after 6 years without any bmx, so while I may be young I'm technically quite new to bmx. He still didn't believe that he could do it too.
36 isn't old by any means. Theres many 36 year olds who run for an hour and a half each day. Riding a bike, any bike, is in no way more demanding than running. As for looking foolish? I wear lycra on my roadbike and honestly a fighting fit 22 year old looks more foolish in lycra than a 36 year old on a bmx. Pretty much everyone looks foolish in lycra. If people well into their 50s are allowed to don lycra and go for saturday rides every week I don't think anyone can begrudge you a little bmx.
Go for it man. Just have fun.