It's not pedophilia, but it's also usually not healthy.
A 30 and a 40 year old are both adults, and despite the difference in their age, they should both be able to function at the same level of mental maturity.
There's a big difference between the mental maturity of the average 15 and 19 year old.
That doesn't automatically make it bad. If a 15 year old mature enough to be in a relationship with a 19 year old, then it's no big deal. It's just that most aren't.