Depend a lot on the variables.  Breed of animal, how well is he growing, how hard is he being pushed to grow etc.  assuming he's been vaccinated properly and growing properly, he can serve and settle a penmate heifer at around 8-9 months or possibly a little earlier if everything 'clicks' just right.  

If you're wondering if he'll be old enough for next breeding season ...  it still depends on how well he grows, and how hard he's been pushed.    Good rule of thumb is don't count on a young bull in the first breeding season for settling more cows than the number of months he is in age.  Probably not very reliable if the bull is under 18 months at turnout though.  And that number is the max for the entire season, don't add one more cow for every month for the duration of the season.   If this animal isn't a perfect specimen, you probably should give him the benefit of the doubt and decrease the number of cows he's exposed to.  He still needs to recover and grow if you intend on using him in the next season.  Put him on too many cows in the first season, and it will most certainly play a deciding factor in how many cows (if any) he'll be good for in the second year.

as mentioned above, test your bull(s) before turnout