Season 3 Mike is 17

I was watching Family Ties, Part 1 from Season 4 a week or two ago, and Jason mentioned to Carol that she was going to be 18 that following year. The whole episode was based around Mike wanting to be treated like he was an adult since he graduated high school and was also 18. So he and Carol are only a year apart making Carol 16 in season 3. Ben on the other hand, I think he was only 11

Not sure about Ben either. As far as Carol and Mike, there is an episode in second season where Carol has the chance to skip 10th grade. Mike is distressed, because then they would both be in the 11th grade.