We know very little of Ben 10's timeline. Note that the original series ran for four seasons, if I'm not mistaken. Assuming approximately one year per season, that's four years, yet Ben was 10 when it started and 11 when it ended. Time in series is not the same as in real life.
I am not talking about time in real life, I mean that we can right the history of the world of Ben 10 . in other wikis they are wrighting the history, in avatar wiki, they use the time of sozin comet to know the history of the events. in the world of ben 10, The world has the same history, it will be easier.
In avatar wiki they gussed alot of characters age, most ages wasn't mentioned , It was only gussed. I think it's easier to guess. At Ben UA the tecnology (mobiles, TV, cars) looked like 2010 and the beginning of 2011 tecnology. In OV( I don't watch , tell me if I am wrong) Ben had a smartphone. which make it easy to guess the year that Ben 10 UA and OV happened in(2010-2011) and in AF it's (2009) , The original series was in 1999.