It's probably true. Pablo Ozuna was blocked by Luis Castillo on the Marlins and our very own Juan Uribe on the Rockies. Ross Gload was blocked by Todd Helton as previously stated. After awhile, they never get a chance and are deemed "too old" for the majors.
I really think Ozuna's problem was when he went from 26 years old to 30 in one day. He lied about his age and no one wants to bring up a 30 year old instead of a 26 year old. They probably figure he has already reached his peak and will be on the decline.