The age limits for the RAF are under 24 when starting IOT (Inital Officer Training) as pilot, and under 26 when starting IOT as Nav. From when you first walk into the RAF careers office, it takes somewhere around 5-7 months before starting IOT if all is successful. There are 6 IOT courses a year. The age limits are extant, getting an age waiver is v. difficult, if at all possible.
The age limit for the Navy as pilot (I'm guessing Observer too) is 26, and I guess this is also when you start training. I think the Army pilot age limit is 28 or 29.
In the RAF and Navy you must be a officer to be a pilot, in the Army you can be a NCO, but first must complete 2 years as an NCO in the Army.
If you join the RAF as NCO aircrew, you can transfer to pilot (means doing IOT) up to around 31, but there are very few people who do this (I believe 4 a year).
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