It's not too old, but it may take a lot of extra work to get her bred. We see dozens of older maidens each season and I personally enjoy working with them a lot, but one of our biggest challenges is educating owners on how involved it is to breed maidens over the age of 6. Each year that goes by is more of a challenge but no, it is not too old. You may even get lucky and have one of the girls who catch on the first try, no worries and make us feel silly for having spent all that time telling you she won't catch right away! Forewarned is forarmed though, right?
The best advice I can give you is for you to work with a reproductive specialist, not just a vet who does a little repro on the side unless they have a good working relationship with a specialist. The second best piece of info I can give you is to start EARLY. Like now. We have already begun soundness exams on our maidens for this season. You need to characterize the cycles well and make sure that there are no underlying health issues (culture, biopsy if indicated on ultrasound) at all before you start breeding.
Are you thinking of AI or live cover? This will affect your time and money as well, because often you need to pinpoint ovulation and monitor the mare so if you're going to ship her to a stud, you'll want to make sure they can ultrasound her post breeding to make sure she ovulated when you thought she would and then she'll need to be checked at day 16 to see if she caught. We like to check at days 21 and 45 as well to look for twins and to make sure she hasn't lost the pregnancy as well as monitoring progesterone levels so you can be sure she'll hold on to the baby you've spent so much time and effort on so far.