Illegal: "we think you're too old to work here"

Legal: "we don't think you have the energy/disposition to be devoted enough to the company to put in extra effort when needed"

Honestly, I'm surprised they even gave a reason. Our HR department has an email template that we must use, which is something along the lines of "we picked someone else, but we'll keep your CV on file, in case anything else opens up". You don't have to give a reason, so if you do, you're only opening yourself up to liability.