P,
As one who did it in the late 1980s (left the RAF, joined BA at age 38, retired at 55 - 6 months "too old" to stay
, now in a well-known LoCo until 65
):
It was worth it for me - I had a command at LHR in 9 years and could have had one at LGW after 3 years as a result of the Dan-Air acquisition if I had bid for it. I could also have had a 744 command for my last 3 years if I had bid for it.
However, if the current situation vis-a-vis the pilot demographic profile had pertained then I would not have joined which means, of course, I wouldn't do it now if I was magically 38 again.
I think you'll have a far more rewarding and progressive career if you don't join BA - assuming that you wish to go for promotion beyond being a line captain.
Timing is all. Now is not the time for mature joiners IMHO.