First, Keep in mind this was written by Vivek Wadhwa, who has a history of attention-seeking articles and statements on social issues in tech.
Second, while the startup scene may follow this trend, the larger employers of the SF Bay Area are not staffed with 20-somethings, they're staffed by all age groups.
Also, in the startup scene, since new upcoming startups are mostly headed by young newcomers, I guess they would prefer working with the same age people to avoid criticism from an experienced professional. An experienced guy would probably be not as easygoing as them. However, I think, in the end its their loss, one can always learn from others experience.
I don't think experienced guys would find learning a new technologies hard, in the contrary it might be easier for them.
The only point where the young people could be a better choice for a startup is the working hours. Assuming experienced professionals are married and settled, working at a startup might be difficult for them.
Second, while the startup scene may follow this trend, the larger employers of the SF Bay Area are not staffed with 20-somethings, they're staffed by all age groups.