To answer some of your questions (hopefully!):
Yes, it is a young mans (or womans!) game. You will be paid National Minimum Wage while you are a cadet, and be expected to work 6-7 days per week, as well as spending approx. one month in every three working in Sweden, along with long / antisocial hours. Atlantic will not discriminate on the basis of your personal circumstances, however i really don't see how you can make it work if you have a family. It's hard enough to do it with a supportive boy/girlfriend.
The chaps in Leeds will have a better idea than me, but traditionally Atlantic have 2 intakes per year, one in the new year, another in late autumn. Having just taken on a new cadet recently, I cant see them taking another for at least 6 months. However, for a start date in 6 months, you might start the application process in 3-4 months.
Atlantic will accept people up to ATPL theory / CPL stage, however you will be expected to do the same length of time as a driver / dogsbody as if you had just a PPL. The reason for this is to make sure no one "jumps the queue" on the basis of training they have already recieved. It also allows them to standardise the training recieved. The usual timeline for a cadet is to start ATPL theory after 4-6 months, then start CPL/IR about a month after completion of the theory exams. Usually it takes about 20 months from PPL - fATPL, and after that you will probably be on the next type rating that the company runs. You will get a say in which fleet you would prefer, but it is very much "needs must" as far as crewing goes.
Check out the other threads on this subject, they are out there.
All the best
TTR