Ax, as stated on another post, is very young compared to elfangor. for half of his life, he was raised as an only child, since elfangor was long gone deep into military affair far away in the galaxy.
So, i reckon, when you look at it in a human family perspective, Ax is like 13-16, while elfangor is an older brother who is about somewhere between 30-40 years old. its possible, there are some siblings out there who have that big an age gap.
andalites do live very long anyway, right?so when they are both in their hundreds, no one would probably be able to tell the age difference physically. they'd both be raggety old codgers.