Perhaps it is because
A, men over 30 tend to be a bit more polite than the average 18-20 year old and tend to be able to speak in whole sentences as well. However this is not always the case and there are many polite 18-20 year olds who don't speak 'innit' speak and plenty over 30 who do
B, Men over 30 are less demanding and don't want to change position every 10 seconds and don't try and pretend they are starring in their own porn film like a lot of the younger ones do. They try to fit the whole likes list into a 30 minute booking which is just not possible and then feel cheated they didn't get to have not only a GFE but water sports both ways, domination, humiliation, strap on, a toy show etc in their 30 minutes
C, They tend to want to fuck for a lot longer than older men who have learned there is more to sex than just shagging someone
All of the above do not apply to everyone of that age but the majority of the younger guys who punt and it is hard to guess if someone is 19 or 24 at times so it is easier to have a cut off of 30