I love Tanner but he is simply not as good as Taysom at this stage of his career. The program is always more important than any individual. If we lose focus on that point alone then we are back to the Jake Heaps/Riley Nelson days again.
Having Taysom with Tanner at the same time would be an ideal situation for the program even if not ideal for Tanner.
I personally don't put a lot of emphasis on age of a player as I know if you are good enough to play in the show then it doesn't matter how old you are when you start your career. It matters with draft position possibly but not in making a career in the NFL.
Think of the FSU quarterback Weinke who was 28 after a stint in baseball minor leagues. Drafted in the 4th round and played 7 years in the NFL after winning every college award as a QB his senior year.
Do what is best for your program should always be the emphasis rather than for an individual if you plan on coaching long term.