Indeed, there's nothing wrong in starting when you're 40. in fact, one south coast school I spoke to a few months ago said they frequently had career-changers in their 40s going through the motions...
...but my confusion is why airlines don't see the benefits in going for an oldie.
As Whirlygig mentioned, there is a concern that an individual's learning skills are shown to be inversely proportional to age - which may be true with certain mature folk. However, as a lecturer with the Open University I tutor a very wide range of students between 20 and 70. Believe me, there are some seriously smart, able people out there across the age range.
At 40 you're no where near past it - well, I'm definately not...