There is definitely a higher expectation for someone over 50 and with someone with an advanced degree. It's both unfair and understandable because the subconscious implication is experience and advanced degrees cost more, so they will be subject to intense scrutiny to make sure paying someone a premium is worth it. Basically, a guy over 50 with a PhD better be a convincing Donald Knuth or good management material.

Also, I think team leads tell themselves they can mold young developers in their image. Not only are you getting a developer at a discount, you're getting a potential clone. What's not to love?