The unfortunate fact of female sexuality in our society is that too-young is very quickly followed by too-old - to conceive, or even to attract many men in the first place. 'You're not allowed to date, young lady' (from conservatives) or 'You're too young to settle down' (from liberals) segues almost instantaneously into 'What, no boyfriend?' The elusive window-of-opportunity - not the Pill, not the tendency of 20-somethings in crappy relationships to end those relationships - is the problem.
Solutions? Since the biological clock is unlikely to budge, it's clear we have to look, at least in part, at the younger end of the spectrum.
As it stands, all long-term romantic commitments begun prior to age 30 are viewed as having rushed into things. Without reverting to a system where women are stigmatized for not having settled down by 21, we could shift to one in which 23-year-old couples wouldn't be treated like experimenting middle-schoolers. I wouldn't suggest encouraging those who wouldn't do so otherwise to marry or similar at 20. I would suggest removing the stigma that says that to be well-educated and impressive and so on, you have to find 'that special someone' at 29-and-a-half, marry at 31, and reproduce before (horrors!) 35. I'd instead encourage the happy couples 18-25 that exist anyway not to end their relationships simply because 'there's so much more to experience.'