Only you can decide that. We did it when our oldest was about 7 and our twins were about 4. (or maybe 8 and 5, I forget). We found her through either Sittercity or Care.com. We were cautious at first. We had her come to our offsite rental house and we sat and talked to her for a solid hour (plus the interview we did with her before even getting there) just to make sure she wasn't an ax murdered or a nut job. We felt comfortable with her and went out for a date. The kids LOVED her. Long story short, 7 years later she's a family friend and the kids still hang out with her whenever we go to WDW, even though my oldest is now 14 and the twins are almost 11. I don't even think she "babysits" anymore, she's now married, works several jobs, etc...but the kids have grown to love her and she loves them, so she's a part of every trip. She's a CM, so the kids love hearing stories and gossip from her.
Not at all saying every story works out like that, we got supremely lucky. But if you think you may be OK with the concept of having a babysitter, I'd recommend Sittercity or Care. I do recall that when we were searching, we saw a lot of late teen, early 20's young ladies who were CMs that listed on those sites as babysitters. Or if you can use the service WDW offers, then I'd be cautious but still be willing to give it a try.
