A friend of mine did not run in HS or college, but started training for the sprints on a lark after watching the Rio Olympics. He unexpectedly stuck with it and somehow pulled off an 11.4 / 23.3 / 51.9 in 2018 at age 30.
He lost interest after that but started training again last year once he heard of the postponement of the 2020 Olympics. He says it's "been there, done that" for the sprints, but he's going to give the 1500/mile a shot this time around because it's easier for older people to succeed in. He started slow to avoid injuries, gradually built to 25-30 MPW, added in some intervals, and ran a 4:40 indoor mile earlier this year. He's planning on bumping it up to 50, then 70, then finally to 90-100 MPW. He's not overweight, has stayed healthy aside from some occasional minor niggles, and wants to run sub 4.
I told him that he'll be lucky to get a sub 4 1500, but he insists that a sub 4 mile is "within the realm of possibility" for him even though he acknowledges that it's unlikely. He says he has the speed for it, and that running a sub 4 in the 2023/24 indoor season at age 35 isn't out of the question since a 41 year old has done it. With that said, how close do you think he'll get before he gives up, gets injured, or simply becomes too old?