I'm not advocating implanting heifers, but it can be done between 30 days of age and weaning, without significant adverse effect on reproduction; the earlier the better. Ralgro and Synovex-C(I don't know about any others) - both have FDA approval for use in heifers. Studies of heifers implanted with both of the above, done as far back as the early '80s, showed no significant decrease in fertility/pregnancy rate, and implanted heifers had larger pelvic areas at 1 & 2 years of age than their non-implanted cohorts(easier calving, anyone?).
Again - I'm not recommending implanting heifers that you PLAN to retain as replacements - but if you do implant 'em early, and they grow off well, you don't have to send 'em on down the road, just because you implanted them.
I know I've kept some implanted heifers from time to time, and can't say that they were any less fertile or productive than non-implanted herdmates. But, most of the time, those that got an implant were less-than-ideal candidates from the start, or were born really late in the calving season, so they weren't likely to make the cut, implanted or not - but the extra growth they put on paid dividends when they went to market.