I've had this issue myself here on /. a few times in the last 2-3 years.
Here's my current take on it:
People discriminate based on age, in any field or situation. That's simple psychology. You can tip the reactions in your favor, based on how you behave. I'm skinny, move a lot and wear a relatively up-to-date hipster / better-dressed nerd mix of clothing and my basic temper is sanguine, so people usually judge me roughly 6-8 years younger than I actually am. That does help me when trying to get a quick hire in the webshop next door, although that is getting more difficult in certain ways.
In the field you're easier in for a cheap quick hire if you appear young and nimble. Emphasis on cheap and quick. Easy in, easy out, no hurt feelings on either side. At a first glance, getting such a gig is definitely more difficult if you have a deer-gut, are approaching your 50ies and looking it too.
Then again, take that same deer gut 50ies body, dress it in a good suit and a well chosen shirt and tie combo, adjust your behavior and your speaking a little, perhaps take some training or stage classes, print some neat business cards with "Consultant" written on them and your salary instantly rises by 15K per year easily. Try that as a mid-twenties guy - it's going to be very difficult.
This only starts to work in your favor once you've got wrinkles and gray hair to show. I call it the 'gray-hair-bonus'. You need one guy from that camp for every contract worth 100k and up. They are indispensable, especially if they can talk and have the decades of experience to back it up. I'm turning into that sort of guy and helping the transition with some extra 'finally-grow-up' efforts. It does magic to my rates. And it's simply that I look the age that make 50% of all that possible. I just have to get used to letting that fat student kid do the setup of the next server, even if he makes tons of mistakes ... after all, I'm there to help him out if he's in a jam. But forcing yourself to keep your hands off is a bit of a challenge, I do admit. :-)
My 2 cents.