The vaccine is going to be deployed to communities at risk first, you can't test vulnerable populations via volunteer challenge trials.
It won't hurt the at risk groups to start up high volume manufacturing earlier (it will likely help them), and there is a benefit to everyone each time anyone is vaccinated (the reduction in transmission efficiency).
Optimizing deployment of the virus to bored 20 year olds may be popular among bored 20 year olds, but it isn't the best public health strategy.
Do you think the vaccines being distributed right now were manufactured more than a few weeks ago or something?
Remember it wasn't a given that mRNA vaccines were going to work, we were lucky.
What proportion of approved vaccines in general have such radically different responses in different age groups or demographics?