There'll be some far more experienced beeks along soon, I'm sure, but in the meantime...
Don't mark with this years... If it's a prime swarm, you can assume it's left its parent colony with their queen. That queen will be (usually) at least last year's, or older (no way to accurately tell). Only way she'd be this year's queen is if she was a virgin queen, usually (not always) as part of a cast swarm.
Short answer is that you can't know for sure about her age. I collected a swarm last May. Queen was prolific layer, even into spring. Then the colony superseded her... They'd decided she was past her best for whatever reason!!
The reason I put '(usually)' etc, is that I've found that bees always find the exception to the rule!
T