Training at 40
I was a mid-life career changer - trained aged about 35, first job turboprop FO at 38 and still there aged 40 so it can be done and indeed I know another guy who also went through OATS around the same time as me who got Jet2 at 37 ish. Since joining my outfit I have actually come across a now training Captain who was a TV repairman for 22 years, first commercial job aged 42 and he's now a line training Captain on the Dash 8 so that's probably a salary around £70k ish so you can do quite well even doing this later in life.
On the other hand I do think that it is harder to get a job at 40 than it is at 20 assuming similar grades/ability etc. For that reason unless you have significant private flying experience or are looking to get a job instructing etc I would strongly advocate integrated training if you are older to improve your employment prospects with airlines.
As a lot of people have said on here I really think a lot depends on how much financial pain you can take - I gross about £32-33k a year, in my last year IT contracting I made £119k, even in my permie IT career I wade roughly double what I earn now. It can be very frustrating not having very much money at this age and with no sign of the job market improving for a year or two yet there is simply little chance of advancement - in short I'm stuck on £33k until either people start leaving the company and I get command or until I get a job with a better paid employer.
Despite all that I am much happier overall than I was doing my old job. With two years commercial flying now I still love it much of the time. I'd say I still like it enough that if I won the lottery I'd carry on doing it (albeit on a part time basis) but it's not as great as a day off anymore (which is how I felt initially!)
Not sure if this helps or confuses, but wish you luck anyway and would be interested to know what you decide.
Desk-pilot