I've seen men starting to lose hair in almost all ages. Why is the onset of hair loss in men different? Is there a normal distribution?
Mostly it is because of the scalp and a decrease in the hair follicles ability to maintain production of sturdy hair (keratin along with melanin for hair color). In some cases, it is also genetic such as in Male Pattern Boldness.
Hair thickness changes with age. Hair is made of many protein strands and a single hair lasts between 2-7 years on your head. The hair then falls out and is replaced with a new one. How much hair you have on your body and head is also determined by your genes. With age, the rate of hair growth also slows down. Source
I would say however that the major impact-factor ought to be testosterone production especially in the case of MPB. More specifically DHT (Dihydrotestosterone) which is a derivative of testosterone. The amount of DHT is higher in balding scalp than non-balding ones.
In addition, it has been shown that the number of DHT receptors on the hair follicles also increases in balding scalps. Also, blocking the conversion of testosterone to DHT delays progression of hair loss and onset of MPB. That is why DHT is usually held responsible for miniaturization of hair follicles leading to hair loss, and this can happen at any age based on the DHT amount, but usually happens as we get older. Source
Testosterone is mainly found in man, but women certainly do produce it, even though it is present in minuscule amounts. Under normal conditions, women have a very small fraction of the level of testosterone that men have. However, even a low level of it can be converted to DHT and trigger hair loss in women. Source