1) They are typically more expensive than "market" for the same role a younger person can fulfill at acceptable tradeoff of competency - higher salaries, higher related costs like healthcare for a family, expectations around retirement programs, etc.
2) They are less flexible - they are less willing to relocate, they have kids to pick up instead of "beer hour bonding", unwilling to run the same 60 hour gauntlet that a 25 year old can etc.
3) They have less primary & secondary education relevant to today's enterprise issues. In the specific case of "marketers" - like TryOldster is pitching - the best people will learn anything, but anybody over 50 years old spent their professional training + formative 20s thinking about television, radio, and print - not paid search, mobile advertising, and social.
That we're at the top of HN again with another "hire older people" post (recently we saw OldGeekJobs) demonstrates there's a huge unmet need within this cohort.
I don't think a job-board is the right solution to this problem because the pitch on TryOldster does nothing to alleviate the three principal concerns.
My unasked, probably asinine business advice would be to turn this flow of traffic into a training / education platform where you can VALIDATE and address the very real, foundational concerns of hiring managers around this cohort, and suddenly you've got a machine that can get motivated people trained and placed.