As of approximately 1 month ago, it is now illegal for employers to discriminate on age.
Whilst this may be the law the problem arises on the strength that if an airline wants to employ people younger than a specific age (35 seems to be popular at the moment) then they will find alternative reasons for the rejection.
Your problem comes with age vs experience, where airlines are looking for experience/hours commensurate with age. Do not lose heart, there are many people who are finding positions who others would consider 'older'.
In some peoples opinion they would rather see maturity on the flight deck as opposed to the younger generation.
At the age of 31 you are well placed to have a reasonable combination of maturity and also being attractive enough to airlines, I know BMi prefer pilots below 35. You have nothing to worry about, go for it and don't look back.