Jam,
I am by no means an expert on this, however, I would go for it. Obvously there are a number of factors to consider and it depends on your circumstances, for example are you a home owner, do you have a family and other responsibilites etc etc. How will this affect your training program?
While I was doing my APTL's there were plenty of people well into their thirties and still pursuing their ambition. I am sure that some people will argue that the younger you are the more chance you have of getting employed as a professional pilot, however, I think if you are determined enough you will achieve what you want to achieve, no matter how long it takes.
With respect to your qualifications, the better they are the better it looks on your CV, I guess? However, if you have been through the professional pilot training and satisfied the criteria, and obtained your licences, then that shows you have met the standard required. However, on saying that, I am not sure how airlines assess your qualifications. One thing I would say after going through the ATPL's, is, it would help you to be proficient at basic science and mathematics. Some of the modules contain scientific and mathematical subject matter, and even though it is not hard, it may come as a bit of a shock if you have not looked at a maths book for 15 years, which is common in a alot of cases, due to the fact a lot of people fufil this ambition later in life, and have been out of school for a good 10 or 15 years. I guess it is just a good pre-requisite on starting your training.
Good luck.