I leveled 3 pets to 70 with me, my cat was 8 when I was 10, my wind serpent from 46 both and my bird from my 66 her 64. I had a wolf from level 10 - 65 that I abandoned for the bird.
They are all 70 now.
I found out that if I go with purpose of leveling pet it seems like forever and they never gain levels or loyalty but in snail pace.
What I did was simply forget about leveling&stuff and go my merry way with pets by my side.
If I went soloing I took the highest pet and tried to watch my aggro. Otherwise I did lot of instances, they tagged along and it was amazing how fast they gained levels and loyalty.
Last few levels were the toughest I admit, because I was 70 and they got stuck one at 68 and two at 65, but few Underbog and Ramparts runs to help guildies and SV, TK, and Slabs after that got them all to 70 without me noticing much effort in leveling them.
Edited, Nov 29th 2007 10:17am by Sethy