There is no age limit as to do so would be ageist and against Irish legislation. There is a minimum age due to applicable licencing age with the IAA. They have certainly taken people on well in to their 30s previously, if not older.
As said elsewhere above, they have usually taken on people who have shown commitment and determination throughout their lives/careers to date. I imagine they want low risk people who are technically minded with more than a solid chance of passing the difficult course in Jerez. Flying experience is not necessary, but an interest would help etc
For salary, direct entries if I recall started on €77k, but cadets will definitely be on lower than this I would imagine. Maybe someone can clarify. Also not known what the "small allowance" while in Jerez would be.