i'm not sure, back in the 70s and 80s the champions were way more tougher then they are today but they got shot at a younger age compared to todays champs i guess it shows the difference in quality of fighters in previous eras compared to what we see today, take marvin hagler i'm not sure if theres any one at 160/68/75 in 2days era that could hang with a 33 yro hagler the night he fought sugar ray
I went for the first option but what I think what you are seeing more now is the best fighters who look after themselves, carrying on being competitive at the top level of the game and even improving a lot older than what most people expected. Due to training techniques, conditioning, knowledge, lack of young talent or whatever.
On the technical side of things, you can improve. But there is a point where being human and ageing affects phsyical performance, and i don't give a **** who you are. Boxing is not 100% physical, it is a skill sport aswell.