Has anyone been able to test if Endurance is retroactive? For instance, if you start with 1 END and level to 10 (and then perk to 10 END), would you have the same HP as someone who started with 10 END at level 10? I am waiting for my video card and hoping to get this answer for my start.

You don't get health from levelling up, only your endurance stat and certain perks. So it does not matter.

Not sure where you're getting that from. Each level up definitely gives you more health.

However, from what I'm seeing and reading, endurance is in fact retroactive. Somebody with PC tested it using console. A level 50 with 10 starting endurance and a level 50 with 1 starting endurance that was raised to 10 should have the same HP.

However, remember not to take the endurance bobblehead until you hit 10. Bobbleheads can increase stats above the maximum of 10.