Pragmatically means testing, checking, dealing with appeals, it all costs money...probably more money that saying if you are over a certain age you get these benefits. Its actually more cost effective for the govt to allow the small percentage of folk who could afford to do without the benefits, and who claim for them (many do not), to claim for them on the basis of age, than to nitpick and means test. Again, many of the pensioners who get free prescriptions because of age, will also be entitled to them because of the medical conditions that they need the prescriptions for and again, the processing of the prescription payment costs money.
I think this is a bit of a hot air balloon....you talk about doing it, people get upset then hey presto the kindly government "decides" not to do it and we are all delighted! This is not especially a poke at the coalition or members thereof, all political parties do it.