By “age differently”, I mean “don’t age at all”. Muppets are essentially vampires who are stuck at whatever age they are made at. So Elmo will be a forever-child, even if he lives to a 1000 years old. So it would be weirder if Elmo ever got a significant other, but Ernie having a 3 year old Muppet girlfriend may just mean she was made as a 40 year old fully formed adult and that’s what she will always be.