At 23, the thought of dating anyone under 21 is not ideal to me.
<Playground Bullies> of Zul'jin (4/13H)
i5-3230M @ 2.6ghz - 16gb DDR3 - 1TB HDD - Dual 1gb 660m in SLI
i5-3570k @ 3.4ghz - 8gb DDR3 - 128gb SSD Boot Drive / 1TB HDD - Radeon 2gb HD7700
You can't put a number on relationships. There is no math. There is no formula.
Whether she is 17 or 18, if you find her interesting enough and attractive, then there's nothing holding you back from pursuing the chance at a relationship with her; nothing, but the taboos of our society.