It’s not her age, it’s her policies.
Young people didn’t like Obama because he was younger or black. They actually believed that crap he said in the campaign. It was something they had never heard before in their lives.
One quick example: Obama said no mandate was necessary, people would buy healthcare if it was affordable. Clinton said a mandate was necessary.
Obviously Obama lied (or “evolved,” or whatever we call lying these days), but that’s how you talk to young people. You tell them the truth (no mandate is necessary in single payer, he was right).
Then you wait for them to vote for you before you disillusion them.