Hi! Ive had many mice over the past few years, so maybe I can help.
I've seen tumor removal done on a very young mouse... But you're correct, tumors generally come with age and/or incredibly bad breeding practices. Mice don't handle anesthesia well, so that's one concern. Another is that mice are so tiny that finding a vet who can perform such a delicate surgery can be tricky. And another... the tumor could come back so quickly that it will just bust through the old surgery site, meaning the vet's time, your time and money, and the mouse's healing time was all for nothing.
If your mouse a tumor (mammary tumors are so ridiculously common), it's best to take him to a vet just to make sure that it's not an abscess or hernia or anything. While there you can discuss quality of life and what to do when his time comes. I've had mice live with tumors for many months and pass of old age. It really just depends.
Treating mice for an infected wound or a URI is generally very easy and pretty cheap.
I hope I answered everything!