Sylvanas is/was a High Elf. Those have a different lifespan than Night Elves. Night Elves were functionally immortal due to the World Trees influence. High Elves had a prolonged lifespan due to the Sunwell, but they were still mortal. Difficult to compare the two.
I’m really not overly fond of the way Blizzard has been handling the age thing in regards to elves. It sounds totally plausible to me that Sylvanas was the bestest, most amazing and incredibly awesome Ranger at age 20, because that’s more or less the quality of Blizzards writing lately, but it still kinda… irks me that they’re treating such a long lived race as basically humans with pointy ears.