I can't see what they have to lose by employing older Pilots?
An initial type rating is challenging and needs to be completed within a finite timescale to a better than acceptable standard - if an employer is prepared to bond you for this, they require good odds that you will succeed in their timescale and to their standards. Youth presents lower training risks.
Younger employees are easier to indoctrinate, absorb information faster, are more mobile and are less likely to leave as seniority grows; many BA pilots will have spent their entire working lives there by the time they retire.
If you have the relevant licences and medical I don't see why anyone should be deemed too old for employment as a pilot.
Because relevant experience is always measured commensurate to age - it's not as simple as just meeting the certification minima to qualify unless you are at the start of your career and you are young.
Do you really think the passengers care about the crews age?
Yes, I think they do. I have heard comments concerning the youthfulness of both cockpit crew members and this is directly related to a layperson's perception of my earlier point about age being commensurate with experience. Employing older pilots however, with little experience would hardly compensate.