I wouldn't breed them till they reach at least 65% of their mature weight. Their age at breeding can vary a lot because of how fast they grow, how well-fed they are, and many other environmental factors. As a general rule, we breed our heifers at around 14 - 15 months of age. Good luck!
The Vets and experienced cattle friends, want the heifer to calve after she is two years old at the minimum . Especially if she is a larger breed animal, who will take longer to get bones solid and fill out. But even the little Dexters were not recommended to be bred young. Still wanted them calving after reaching a full two years old at the minimum. Takes too much of the nutrition away from a growing heifer, to be supporting a developing calf at the same time.