It used to be a manor concern feeding a younger steer. Now with the genetics of steers, it depends. Depends on: how big are you expecting him to get, how is he bred/genetics, will your feed program (with or without adjustment) will get him there.
Getting him properly conditioned will be your biggest challenge. His genetics will dictate alot of what you will need to do.
Long story short, buy a steer that will be mature enough if you can. 18 months minimum. That will cut a lot of the hassle.