What rmedgar said is probably the easiest way to tell. The next time you catch a few bass look at them study them keep records of the weight and length. There's a relative weight chart on here somewhere it tells you if your bass are the right weight for their length. If I find it ill send it to you.
Also you ask about juvenile fish your yearling bass tend to stay together for the first year so if their small and cruising the bank together their probably yearlings.