My boy Clarence, has a collapsed trach (since he was very young)& enlarged heart...he's 12...I think it's his bad attitude that keeps him going!
My sister has a 14yo who has had seizures since he was about 4 & Charlie is still kicking.
Then you hear about a, seemingly, healthy pup who just fades away at a very young age. My suggestion would be to just spoil them rotten from day 1 'cause you never know when God is going to want them back.