Actually, they are very strict about moving to the next age group. The rule is that you must be within 30 days of the birthday to move up.
You can ask, and the actual answer would depend on how full they are and the ages of the kids in the teen area. Remember that there is a huge difference between a 12 year old and a 17, and that one of the benefits of the teen area is "no little kids." They do try to keep family groups together when possible and to keep parents happy...so it is not a "no way" but a "don't count on it."
Even if they deny the 12 year old admission to The Stack, there are lots of places that the two of them can hang together--movies, deck 9 stuff, pool, arcade, any "family" activities, etc.
My 12 1/2 year old was told "No" to the teen area (it was still COmmon Grounds then)...this by phone without ever meeting her or making an assessment of maturity. This is a girl educated primarily at home who takes classes at the local university. Since she celebrated her 13th birthday in The Stack and has had one other cruise as a teen, I'm actually glad they denied her at 12 1/2. It was much more special to have to wait.