A quick short list I came up with... Interpret how you will. If anyone feels the need to go beyond top 10 then be my guest.
Below is a list of the top 10, showing their ages when the broke 3:30 for the first time, and their ages when the broke 3:30 for the last time. Age span at the end.
Age First Sub 330 / Age Last Sub 330 / Years of Sub 330
El G 22 25 3xx
Lagat 26 29 3xx
Morceli 22 26 3-4
Kiprop 23 Still Running Still Running
Ngeny 21 22 1>
Farah** 30* Still Running Still Running
Cacho 27 27 0
Baala 25 Same 0
Komen 22 Same 0
Ramzi 26 Same
Mo Farah is the oldest person (age 30) to not only break 3:30 but also the oldest person to do it for the FIRST TIME! That is in the TOP 10, however I think it will hold true going all the way up to 3:29.99 on the list.