Age is a significant and proven factor in training success, and not only within aviation. If you're going to pay for your own training, and are prepared to continue throwing money at it until you pass, that's your prerogative. If you expect someone else to pay for it, you must also expect them to minimise the risks to their wallet. Employers don't have unlimited money, or time, to train their new pilots, and will select accordingly. This procedure will never be illegal, however unfair you think it is from your personal viewpoint.
As the one paying, the employer is the customer. Like you when you wish to purchase something, they are entitled to set their own criteria when doing so.