I don't have any issues with his age, particularly with Benzema and Lewandowski still regarded as one the best strikers in the game - I think generally that retirement date is prolonged in today's game. So, he is not old at 29 (30 in the summer) and IMO it is a myth.
Also, we should be looking to add someone who can have an impact straight away, matching our current spine ages: Bruno, Casemiro, Varane, Eriksen, Shaw, etc. rather than signing a younger one to be a starting striker, who will require further development to reach that level. Osimhen would be a very good choice as he is young enough to be considered a long-term singing and he is also, IMO, ready to have an impact straight away.
If we sign Kane, we would have to add younger strikers in time, but we should not have concerns about our future, there are still plenty of young talents in this squad: Amad, Mejbri, Garnacho, Pellistri, Sancho, Antony etc.