My old man is an IFA and he said that you'd not likely get a mortgage until it was fully paid off -- for me we worked that out to be mid-30s which doesn't bother me too much. I know many people who haven't yet bought a house that age & isn't really the end of the world anyway.
I think you might be ok getting a mortgage around half way through repayments, as then you should be on a very reasonable wage, with hopefully a solid credit rating provided nothing goes terribly wrong.
Do note though, not talking from experience and likely hood is, if you owe someone money you probably won't get a mortgage.