well it depends..
i doubt its old age creapin up on you, cause trust me, ive seen some realy old guys in porn.
well, have you changed your routine?, by that i mean, do do you try to c** and approach orgasm as fast as you can? try to prolonge it, that should help..
also it could be a problem with your (i forget the correct word, lets call it ejaculation nerves, which are located by the prostate, from what i hear, they could have weakened.. or, even worse, sorry to bring it up, may be prostate cancer... dont be alarmed though, because i said that. im only 16 years old, and im just throwing out some ideas.. but i would recomend you go to the doctor about it..