It can be debated until the cows come home whether the 55 limit is to please age legislation or not.
I remember not so long ago when people were worrying about being 29 for a 28 age limit!!
The key is that you prepare properly. There will be youngsters with a lot of time available to keep practicing the aptitude tests, whereas if you just wing it on the day because you've got the day job/family etc., you've got no chance.
Even if you're on the line by say 45, that's still 20 years of service and the last thing BA wants is to employ 500 21 year old pilots all retiring in the space of 5 years in 30-40 years' time.
So if you're prepared to dedicate a massive amount of time to the application process - and I'm not talking about visiting pprune or plane spotting - go for it, but if not you're wasting everyone's time.