What my friend did (she has a gelding with a specific bloodline and personality along with gait that she wanted in a younger foal) so she reserached online and made phone calls and specifically found a young horse that matched her descriptions.
I wouldnt, personally, breed a older mare like that. Yes, the chances of loosing any mare is there when you breed them, but at twenty one I think that the odds could be against her. Good luck on your decision!
Shaneequah, 1998 gaited Bashkir CurlyxArab mare
Treyue, 1999 3-gaited Icelandic gelding
Loki, 2001-2015 Icelandic gelding