My first horse had previously had six foals. I bought her when she was ten years old and at 19 we bred her for that same reason. She had no trouble what so ever. I personally wouldn't want
to be the one to breed a maiden mare much over 15-16 years old. We were more comfortable as my mare had produced six foals in the past.That said another mare here had her first baby at sixteen with no trouble at all.
Just something to think about when you make your decision...
In my case I was bound and determined to keep whatever foal came out of my lovely mare. If it was purple and three legged it was staying. I just wanted to breed her once to have something that could be raised by her (and me) to remind me of her. Her foal is now five years old and he was never taken away from her or technically weaned. He has had as much time to learn from her as I could possibly offer. He takes great care of his momma as she ages (25 this year). While there are certainly some ways that he is like her, I see more of his sire in his build and temperament. So make sure that you adore the stallion you breed your mare to because there is no guarantee that you'll get a carbon copy of your mare by any means! As said in my case I was keeping whatever she produced. But I have had several friends want to breed a mare for this reason and the foals have been sold. One has popped on and off from craigslist for sale four times now and the filly is only two years old. So be sure breeding is the right choice. Your mares build and the stallion come into play. If you are going to sell the pending foal on a whim because its the wrong color or you weren't prepared to handle the up bringing of a baby, perhaps buying a weanling or younger horse that has similar breeding to your mare is best.
In my case, it turned out to be a great decision and I adore that little piece of my mare.