The various and changing timelines and ages are examined (quite well and thoroughly) in the "secret history of star wars" which can be found on these forums if you do a search I guess. Forgive me I can't remember who the author is but he is a member here.
Offically there's 19 years between III and IV.
Ep1 - Kenobi is 25 Anakin is 9.
10 years in between
Ep2 - Kenobi is 35 Anakin is 19.
3 years pass
Ep3 - Kenobi is 38 Anakin is 22
19 years in bewteen
Ep4 - Kenobi is 57 Vader is 41
If I'm mistaken someone correct me as I did this quickly.
Add on about 10-15 years and things should be right.
Definately check out the web book I mentioned above. If I have time later I'll try to look it up.