Hi Tracie,
If their wings are not fully developed yet, than they are probably younger than three months. I have two sets of hatchlings. One set is now almost 4 months old, the other set is 1 1/2 months old. The 4 month old ones have looked like adults and had all their wing/tail feathers now for over a month. So, at 3 months, they were already fully feathered. The 1 1/2 month old muscovy ducklings are about half grown. They are about half way to having all their wing feathers in. They have tail feathers. When they stretch their wings, they still look "stubby" even though the feathers are growing in on them. If you can post pics of yours, it may help with getting a better estimate of age. Plus, I love seeing other peoples muscovies!
Glad to hear she seems to be better. As for the peas, it only takes a couple minutes for them to defrost in hot water from the sink. Don't microwave them as they will get too hot too fast and start to cook. I just pour mine into a bowl, run hot water over them and let them sit in the hot water for a couple minutes. Then drain them, take them out and feed them to the ducks. Works great! And they love them.
How long have you had the ducklings? Is it possible that they may be sick? It sounds like you "rescued" them when you bought them. Oh, I just thought of something else too. They may have a niacin deficiency. Do a search on niacin and brewers yeast. Lots of threads on that. Apparently it causes weak legs. Any idea what they were being fed before you got them? What are you feeding them now? They should be on non-medicated chick starter like Start and Grow, which you can get at the local feed store.