You can't teach an old dog new tricks

An old cliche, and one that could only be used by someone who's never had a border collie.

Like many hoary old cliches, this one has little or no basis in truth. There is as far as I can find in a quick lit review little or no evidence to suggest that old dogs (or indeed people) can't learn just as readily as those younger. It's a common assumption though. I often hear people say they're too old to learn something new, or they can't learn a new language because 'it's too late to start'. It's an easy cop out for people I think, an excuse not to do something they think will be difficult or onerous.

Even psychological journals trot it out. This article for example, claims to describe the reason, without ever providing any evidence that it's true.

Many theories of child development postulate complex developmental 'stages' which are arrived at chronologically and in sequence, where learning and thinking happens in qualitatively different ways once the stage is attained. The evidence for this is patchy at best, and tends to be as well or better explained by a connectionist model rather than a modular model.

Anecdotal evidence from teachers of adult learners suggests their adult students learn much more readily and rapidly than children. This makes sense, especially if you look at things from a connectionist point of view. Adults have a much greater store of knowledge and skills to integrate new information and skills with. Better connected information will be retained much more easily (you can see this in studies of brain damage and memory). Cross domain learning, and a greater ability to generalise and reflect seem likely to make it much easier for adults to learn.

It was long thought that there is a 'critical period' during which children are able to learn language, then some postulated organic change takes place (believed to be around puberty) after which it becomes very much more difficult. The evidence doesn't support the theory at all. It does support the theory that greater knowledge aids learning.

Practical applications
You're never too old to learn.