The steers we slaughtered on the farm growing up were guernsey x hereford (or angus; whichever bull was king of the hill that day on the range we shared with other farmers ). We slaughtered them at about 20 mos. of age and they weighed 800 lbs., give or take.
You may want to go by his physical condition, rather than an age. I sure would want my steer to be WELL covered in the rib area, no ribs showing. This would make him fleshy on his hips and top of the tail area as well. Lumpy tail top with fat globules can show when animal is too fat, learned that in 4-H! Smooth flesh covering of body, with no bones showing or protruding, seems to be about the best visual method of guess-timating readiness to send steers off.
Depending on what he gets to eat, he can gain enough to be done younger or later. My friend had a young bull calf 14 months and steer calf the same age, both Jerseys. They were HUGE! They were on pasture, with a daily helping of corn, probably a couple quart measures for each animal. I took my Dexter over to use the bull, and was quite surprised at how big he was for such a young age. I am betting at least 900#, though bull was probably only 14 months. Towered over the cow, and same age steer calf was almost as big, really well fleshed out, no bones to be seen! They could have been slaughtered then, but waited to get the two cows serviced. Great pasture, with that bit of corn for marbling of the meat, makes for great eating! I have been over for supper a few times, excellent meat!!
Friend has always done the bull and steer thing, with bull servicing his milk cow, then both males go to slaughter for his extended family freezers. He had the next young pair of males in another corral, ready to put with the cow on pasture when he shipped the older males. For him it works. Little to no "bullish" attitude with young age animal, being field kept and gone before he thinks of being bad.
Smaller breed animal like a Jersey, may be ready younger, so you don't have to feed as long a time before taking them in.