Evo,

Just as a salmon or steelhead "jack" is a sexually precosscious male, there are environmental and physiological conditions that combine to mature rainbow/steelhead males at less than two years of age. It's not like a baby being sexually mature because a two-year old trout is not a baby. They stop being "baby" fish by at least two months of age. And a smolt sized steelhead is not 3", more like 6 to 8". There are lots of resident rainbow and cutthroat trout that are sexually mature and spawn at around 6" in length. And precoscious rainbows and steelhead have been directly observed spawning with normal sized adult female steelhead numerous times. They have to be fast and sneaky since there is usually one or more typical size sexually mature males around spawning females.

Sg