Hi Dibs,
The age isn't always an issue to be honest, and I added a young sow to older girlies with no problems.
You can't just put them together though, they need to be "bonded" and that requires a neutral zone with no other smells etc in large area and some distractions like food! and time.
There is thread on the forum for bonding furries, I don't have the link to hand but it's worth checking it out.
Sorry for the short message, I'm at work currently and just finishing my lunchbreak. mallethead