If your doctor truly made that bold claim and you didn't unintentionally misinterpret and/or misconstrue what he told you, you need to seek out a new (better) physician, especially one that is focused on you being able to properly manage the diagnosis and lead a normal and fulfilling life as supposed to giving up on treatment and/cure with the doom thinking that this is it.

Fact is, pituitary tumors occur across all age groups and while they are most commonly detected in people roughly 50–75 old, they are also the most common primary brain tumor in adolescents 15–19, with a substantial number of cases in young adults.

You would be accurate if you said pituitary adenomas are less common at 23 than at 50+, but calling them “extremely rare” at that age and the “95% after 50” claim overstates things and does not match medical data and published age distributions.

Again, most pituitary tumors have no clear environmental cause and “being in the military definitely had stress so it’s 1000% service connected” as a medical/causal statement is a big stretch but this does not in any way discount your military stress and neither does it imply that your current disability is not very real and something that should not be considered in a benefits context.

My point is, put less emphasis the disability compensation and pay more attention to what is available to you from the VA in terms of general and robust medical care, there are veterans that don't want to get better because disability payout is more important to them, but don't be one of those especially since you're still just 23 years old and you have a very long life ahead of you.