I did some work with the DNR a few years back. The only way to really tell how old a fish is is to look at the rings on the scale sample from about the mid back, larteral line up or to remove a vertebra and count the rings. You would need a magnifying glass in both cases. We had one fish that was a 17lb steelhead that was 17 years old and never reproduced , we also had a 15 lb steelhead that was 3 years old and had reproduced. I believe one of the state record steelhead was an 11 year old that was 27 lbs. It's all a matter of forage. Higher energy forage = bigger fish. Salmon and trout that eat smelt, and bloater chubs tend to get bigger than salmon that eat only alewife. Availability of forage also plays a role as well. Lk Huron has no alewife population right now. The salmon ate them all. Lk Michigan has a alwife population but we're missing two year classes. So in Lake michigan, the alwifes are about 2.5-3 inch long (04-05) or 6-7 inches long (99-00 year class).