Though understandable, it is a great shame that they don't think they have anything in common, and you are so right to encourage them to think otherwise. In the first instance, I suggest you consider setting up a separate group for the younger mums — say for teens or under-21s — that they will have no issues with attending.
This way they will build their confidence being in a group and get to know others. Once you have established this group and they are comfortable with each other, you can then try introducing them to the older mums by running joint workshops that they would all enjoy and benefit from. And once these are off the ground, it won't be long before everyone will want to be in the same group.
Tracie Trimmer-Platman is senior lecturer in youth and community work at the University of East London. Email your workplace dilemmas to asktheexpertywn@hotmail.co.uk.