Ziz said:

The problem I have with writing off the age/appearance difference is a proportional one.

Yoda said in TESB "For 800 years have I trained Jedi". So, let's call him 900 years old just to give him some time to get old enough to become a master and teacher.

Now, humans typically live to 80 or so, some pushing 90. That means that Yoda's life span is roughly 10 times that of a human.

Working backwards from that perspective, 25 years in Yoda's age difference is the equivalent of a human aging 2.5 years. For Yoda to look proportionally older from PT to OT, there would have had to have been upwards of 250 years in between. Of course then all the key humans would be long dead (or nowhere near being born yet, depending which direction you're counting.)

Haven't you ever seen people in the last few years of their lives though? Sometimes they can change quite a bit.