You, as the owner, makes that decision. Current management says we cannot afford to keep cattle around til they are 3 to calve. Therefore, it is general practice to breed at about 15 months of age to calve at 2 years of age.
Now, saying that, you can only breed them at 15 months of age IF (and that's a big IF) they are grown out well enough. They should be 65% of their MATURE weight at breeding age. If that means they need to grow til they are 2 years old because of poor nutrition - so be it.
Raising a heifer on poor nutrition, actually costs you more money because you lose a year of production.
On the other hand, if she is being pampered & getting more than she needs & is getting fat, that is not good either.
But, again, you own her, you do what you want to do with her, but are you able to tell what body condition she is?