Young Nick,
Welcome, and well done on considering your situation. The answer to your question is no, you are not too old - I personally know of a lady aged 52 who began training a couple of years ago (aged 50) who is now FOing with EZY.
Of course I am sure there will be others who will read into this a lot more than I can, and may offer some more helpful advice.
But I've heard many successful stories, and I doubt the airlines will mind too much until you reach the grand old age of 50! Remember, this is a personal decision and nobody has the right to tell you you're too old, or don't have the ability to achieve it. You'll only end up having many sleepless nights when you're 80 - what if?
And personally, despite being only 20, I prefer flying with the older chaps
All the best and good luck,