Well if you think about it, Tenzin is the son of Aang and Iroh is the grandson of Zuko which is around the age of Aang a little older though, but it would make Iroh in his 20s because Aang's grandchildren (Tenzin's kids) are at most 12 years old. So because Zuko is a little bit older this would make his grandson a little bit older than Tenzin's kids. And if i am wrong, sorry.