I'm 37 and have not been concerned. If I compare my current work to that I did a decade ago, it's obvious to me how much better I am -- faster, better at design, fewer bugs, etc. AFAIK, no one has turned me down for work because I'm too old. Perhaps they've chosen someone younger and less expensive, but that's fine by me -- maybe not a rational choice, but not one based on discrimination.

Other than code quality, how was I different as a 25 year-old? Much more likely to adopt new technologies, libraries, etc. because they were shiny and hyped. Did this serve me well? Sometimes I got lucky, but I also wasted a lot of time figuring out the hard way that Technology X was way too bleeding edge for production. I was way too willing to go along with (or even suggest!), massive, all-things-to-all-people architectures which looked great as block diagrams but didn't match up with timelines, project risk, etc. Now, I push for incremental change over big bang projects.

One way I've regressed in the eyes of an employer: I'm no longer willing to put in long hours in exchange for vague promises of future pay-outs (bonus, promotion, whatever).