I was thinking earlier that the young adult resistance/ambivalence is a huge problem with the goal of vaccine induced herd immunity. Whatever percent of the population is calculated to need immunity to reach herd immunity, the calculation is based on a roughly random sampling. If the number for COVID is 70% (it could be higher or lower, I don't know), the assumption is that the virus only has 30% of the population to spread to and it is unlikely that an infected person comes into contact in a way that they can transmit it with one of the 30% not immune.
However, with the vaccination rate being so varied by age, I don't think it will work because for most situations where spread is likely to occur, people are having close contact with others that are near their age. College students don't typically invite their grandparents to frat parties.
If you end up with 70% of the population vaccinated but if under 30 is only at 35%, there will not be herd immunity in the under 30 population who spends a lot of time with each other. Based on the trends to date, it will take a really successful outreach program to get the under 30 crowd anywhere near the required percentage vaccinated.