I think your level of formality should reflect the wedding in general and your husband's attire in particular. It sounds like they haven't made a plan regarding the ceremony and reception, so I would definitely not buy anything until they have decided how formal the event will be.
Once you know that, I think you have a lot of flexibility. If the wedding is formal, or if your husband will be in a tux or nice suit and the bridesmaids will be in long dresses, I would go with something long. If it is more casual, then I might go with a knee-length dress, but long is usually appropriate for mother-of-the-groom anyway. But from there, I think you should get something you like and makes you comfortable. I would not feel the need to get something that seems "older" because your role would typically be filled by someone a few years ahead of where you are. As long the dress fits with the formality I think any nice dress that flatters you would work.
One word of warning regarding wedding etiquette: I know a lot of people ascribe by the guideline that the mother of the bride chooses her dress first, and then the mother of the groom chooses something that will be complementary (or won't look horrible next to it in photos). It might make sense to talk to the bride's mom as well.
