Yeah... I usually keep my guys until they retire or lose so much talent that they can't maintain championships and then I look for new talent. I'm not quite to that point (32 years old is my oldest boxer, I'm 34 in the game) but I figured I'd do some scouting, and found that there's just no talent left in the young guys.
I checked the two careers to see what was happening. Both careers have 125 pages of boxers and 40 pages of free boxers, so technically regeneration is working fine and my opposing mangers aren't managing any more boxers than they were at the beginning of my career.
There is actually lots of young talent - each career has 16 pages of boxers 16-18 years old, so it's actually not regeneration that's the problem. The problem is that 13 pages of them are signed by other managers, leaving me with very little free agents to work with in my old career, vs. only one page signed by others in my new career, giving me 15 pages to choose from.
So my opposing mangers are doing exactly what I'd do - drop older boxers with less upside and sign the young talent that has lots of potential. Problem is that they've now signed everybody worth signing and I'm left with basically nothing that's really worth my time and money to train and try to get to championship status. Hum... so in some ways it's realistic because my opponents are smart too, but...but... I don't want to HAVE to steal from another manager to get my next champion. Plus these guys have the tendency to be high in the rankings already, rather than in the bottom 10 and needing to fight their way to the top.
Ultimately I guess we would need for every new boxer 16-18 to be automatically in the bottom 10-15 of the rankings so they'd have to fight their way up, and we'd need to force the other managers to sign guys who are older to keep the balance reasonable for everyone.
Hope that all made sense.