Eternal life (immortality) is useless without a few rules. Eternal prime, Regeneration, infinite memories, perception of time = age 20-30.
The big one to me would be perception of time. As you age, your perception of time changes.. Thats why when you were young, your school days seem to take forever, where now you work, and the weeks/years seem to come by faster. Thats because 1 year when you are 10 years old is 10% of your total life, so it seems so long.. when your are 50, 1 year is 2% of your life, so it seems it went by so fast. As you get older and older, that perception increases.