Why..

In Kh 1and @ they were different forms of Xehanort. In BBS That was the original Xehanort, but in the end he took over Terra's body, making him younger. But then he lost his memories, making the Xehanort we say in KH2 in the flashbacks. In KH1 we saw the Heartless of what people call Terranort (Terra possessed by Xehanort) and KH 2 was the Nobody of Terranort. That is why he was younger in KH 1 and 2 than he was in BBS.

 

Dude, spoilers. Not everybody has finished BBS. Anyways, what TheKnightofAwesome posted explains it, I just changed the text to white since it is a rather big spoiler. Highlight his post to read what he said.

Oh, and here's the spoiler free answer to your question. BBS explains why Xehanort is younger in KH1 and KH2 and older in BBS.

 

Xehanort doesnt even appears on KH1 and KH2,his heartless and his nobody do

 

read the spoiler dude. anyway, yeah, didn't realize at the time I wrote it how bad it was. I guess I thought after all this time it wouldn't be that bad of a spoiler