Hello people,
The correct answer to this question is probably the age at which each induvidual gets into the industry. We must be sensitive to the older people who are self funded and may not start training and get into the industry until well into there 30's if not 40's.
That being said the younger the better in my books but it has its drawbacks. Some and I stress not all 19-early 20 types are not mature enough to get through courses and handle the pressures of modern commercial aviation. This is where an extra 5 years (possibly) of university, travelling or just life learned skills can help greatly. Folks of the 30 plus group do provide airlines with possible stability as they are the types that are less likely to move on from a decent job as they are looking for stability over potential career progression.
In my case I entered aviation college at 23 and did two years. Beat the younger mates in my class but was neck in neck with the older ones. Got my first gig on turbos at 25 and was a skipper on a large turbo at 29. Currently RHS on a Boeing and have 30 years according to FCL to play in the jet world. Not bad.....
Don't rush if you are very young, you have years to play with in my mind.