I have 320 beef cows, and about that many steers/heifers waiting for market.

18 months on a steer is about 1200lbs, pasture fed then grain finished before slaughter. A non-castrated male can get to 2200lbs after 4-5 years. I’d call that an adult bull. They can breed successfully at damn near any age after 12 months, but I wouldn’t call them developed until about 30.