I think that after she left MM has become younger and younger wich has two problems. Young girls just don't have the personality to be intresting or funny and worse they are having to fight the older girls.
I don't think they have gotten younger. In the beginning there was a 12 year old, Asuka Fukuda. Second gen had 14 year old Sayaka Iichi. Goto was 13, Nono and Aibon were 12. After Yuko graduated no one was younger than 13 until the newest member Koharu.
I don't think it has anything at all to do with age. I think the main problem is the number of them. There is just not enough time per episode to focus on all members. Ai-chan has hardly gotten any attention on H!M for a very long time and she is one of the older ones. Erin created a huge fan base last year and she is one of the youngest.
I think sfc1971 was not referring to the youngest member of each gen, who, as you pointed out, were of similar age in each gen. I think that the point was that there was no one older than 15 joined MM after the second gen.
I think that age has less to do with it than the concept of MM being idol training. Looking thru jpop, idols generally have a short shelf life ... Mainly because with these things, after the initial infatuation, it's easier to build negatives against them than positives ... Hence the general negative perception of H!P among some jpop fans.
Another problem is that UFA is a bit too activity oriented. As soon as a group is promoted, minimal time is spent on training, more on work. HP members seem to be some of the hardest working people in the idol / jpop biz. But because of that, additional training is less and you generally see less growth (musically / dancing / etc ) in the girls other than yogurth