You should be totally honest with her. The age at which you had sex for the first time is not the important aspect of the answer. Your experience is at the heart. If you can describe how it made you feel, how you may have regretted the choice you made, and how it affected relationships with other mates in the future, peers, and even your parents, you can convey much more beyond just the age at which you first had sex.
It does not, by any means, mean you will stop your daughter from making a choice that may or may not disagree with your best wishes anymore than watching your parent die of lung cancer may cause you to immediately quit smoking. But it will at least give her something to think about. That's the best you can do.
The worst thing you can do is lie. Lies never help anyone.