Not true at all. Male body continuously produces sperm even into your 70s and 80s - and the testicles and epididymis can only store a certain amount, so if the sperm doesn't get released (via ejaculation) then they will leak out anyway (while you sleep usually). It certainly does not affect your immune system.
On the other hand, ejaculation does cause release of adrenaline in the body (hence feeling of fast heart rate and sweating) which, in someone with heart diease, can result in heart attack - but that's not to worry when you're young la!
