It all comes down to the individual.
There are people that make it through the courses at 18, there are people that make it through at 31.
There are also those that fail at 18, and fail at 31.
The same can be said about almost any age group (even though there are fewer applying from each age group after you start to become a bit older).
On my course the oldest (41 I think) and the youngest (18) both failed during the college stages, however anecdotal evidence is just that, anecdotes. How the actual statistics look I can't say for sure (as I've never seen any).
Why not apply and see how it goes? If you fail the application then no harm done, if you progress further you will hopefully be able to arrange a few study visits and learn more about the job, through that you will hopefully get an idea if you think it's something for you or not.