First time I really felt "old" was when I started hearing/reading more zoomer expression and stuff like that over time (as they're getting older and start to be showcased more on Youtube, Twitch, TikTok existing, posting on forums, some public Discords etc) and not getting a big part of their culture. To me you really mentally become "old" when you're out of touch to the point of not really caring about how younger people see the world, how they act in it. Dismissing concerns, dismissing what you don't understand as being shit or whatever. But that's more acting like a boomer than being old so idk if that counts.
Physically I think it depends for everyone, but I think 50 is a good cutoff where calling someone old isn't insulting. But at the end of the day it really depends on how you eat, how much you exercise, how much you keep your brain active, etc. Some people will look/act/feel older at 40 than some people at 60.
TL:DR : Physically old, 50 is a good ballpark. Mentally old, there's too much variation for me.