I think it would be a management nightmare to breed them at 18 months.. and then it would take them 2-3 years to catch up with the rest of the herd, giving you a 120 day calving season, besides the problem that if you wanted to keep heifers that were born late it would throw you off even more.
Size is the biggest concern for me, I had a late born heifer that grew really well, so I bred her at 12 months and she turned out just fine, though I wouldn't breed any earlier. Conversely, if a heifer is too small to breed at 15 months, I don't think she'd make a great cow anyhow.
I try and get my heifers to about 900 lbs by breeding time... I'm in the process of making my cows a little smaller, so I might need to adjust for that a little bit.