1: Estimating variability in outcomes attributable to therapists: A naturalistic study of outcomes in managed care - Wampold, Bruce E.,Brown, George S. (Jeb) Journal of Consulting and Clinical Psychology, Vol 73(5), Oct 2005, 914-923
2: Huppert, J. D., Bufka, L. F., Barlow, D. H., Gorman, J. M., Shear, M. K., & Woods, S. W. (2001). Therapists, therapist variables, and cognitive-behavioral therapy outcome in a multicenter trial for panic disorder. Journal of Consulting and Clinical Psychology, 69(5), 747-755.
When using age as the predictor I'd imagine time spent practicing is the latent metric actually being measured. You can't have 10 years of experience at age 25. That said it seems silly to use age when time spent practicing is a readily observable value.
Its a mixed bag on age. Younger, I think, is better due to both learning more recent research and also continuous education. Relatableness is a factor. Again, all anecdotal.
Broad agreement among whom? As with MDs, I would expect several complex trade-offs. Particularly between the value of medical (and life) experience against the value of being trained with the current science and treatment methods. For therapists I would also expect more significant trade-offs due to the difficulty of building strong therapeutic relationships across large age differences for some patients. Are you aware of any studies on the issue?