I've read that 2% figure before. Is it an estimate based on a large study, perhaps an average based on data.
I don't see an overriding physiological reason for men to produce less T only possible 'wear' and thus less efficiency in the endocrine system and testes.
Another thing that is interesting is homeostasis. If for a 25 year old that results in, say arbitrarily, 1000 units of T then why is the same system 'settling' for a few hundred 25 years later? Or is it infact trying to keep up but thanks to ineficiencies in the body it just can't.
In a man in prime health there should be no 'inevitable' reason for reduced T, unless there really is some programmed unavoidable ageing process wherein even the fittest, undamaged, stress free man simply loses T anyway, despite there being no outward reason, except i suppose the baseline wear and tear of just being alive. Maybe that's it.
It all seems depressing whatever it is. Accepting T loss as 'natural' would be like not bothering with opticians because eyesight degradation is 'natural'.
You've got me thinking on all this as i'm 36 so it's all good planning for the future!