Having all occupants of the property be under the age of 55 would probably be frowned upon by the association. Before making a committment be sure you have explored these guidelines thoroughly relative to this community.
The building may let people under 55 buy but they may not let them move in...
I would consult the association and probably an attorney one way or another. But if buyer approval is mandated on a sale, the title company would not let the sale pass without a letter from the Condo owners association stating that the buyer is approved.
Buying it in the mother's name and living in it would require the registration of the son with the association - and they may not allow that.