I think Alexis Sanchez is already an old player and he can't be useful for Inter Milan unless they sign a short-term contract with him with a low salary.
When it comes to signing a player, his age does not always matter. What matters most is the value he can provide to the team. Alexis Sanchez may be considered an old player, but despite his age, he performed admirably for Marseille last season. Having multiple striker options is beneficial to a team and can boost their chances of winning a match since the coach will have numerous options to choose from that can fit into the match nicely. I don't see anything wrong with Alexis Sanchez coming to Inter Milan; it will only strengthen the team rather than weaken it.
I would say that it depends on the overall age structure of the squad. Inter Milan has Thuram and Martinez as two strikers who are both 26 years old, which is a good age to also plan the future with them. If the team than adds one or two strikers who are older and more experienced, but still in a great condition, it can be the right action to take to upgrade the team a little bit. That's why I also think that Alexis Sanches would be a good choice since he also said that he is willing to cut his salary in half if Inter Milan takes him back. The financial burden would also be ok then and hence it seems to be a good deal for both of them. Though it has to be taken into account that he got a bit more often injured, which I guess is normal at his age. In contrast to that Martinez is almost always fit.