Where I live the flu jab is free for everyone over 50 this year, but the 'non-vulnerable simply over-50s' (like me) won't get there's until November as they'll do everyone else first (through the GPs mostly).
But if you're offered it you should have it regardless of age. Our local bod in charge of health is urging as many people as possible to have it because it will reduce the flu rate across the community for those who can't have the flu jab. I've never had one before because I don't fall into any vulnerable category (usually!) but this year, I'm over 50, it's free, so I'm going to have it to do 'my bit' to protect others - if I didn't have the jab, and then went on to catch flu and gave it to a vulnerable person, then that would make me much more irresponsible than taking up a flu jab to start with!
The other issue is, of course, if a Covid vaccine does come on-stream later this year (optimistic, maybe, but many drs do seem to think it might happen) you have to leave at least 28 days between a flu vaccine and a Covid vaccine. So that's another reason to get the flu vaccine done asap, just in case.
Incidentally, both my DC get their flu jabs through work, both are only in their 20s (one had theirs this week - and the other actually had flu at university a few years ago, and knows how ill flu can make even a young, fit person...) but their workplaces order enough for all staff, so it's not causing a shortage elsewhere because it's all be planned for.
TLDR: Have the flu jab you've been offered, you're not depriving anyone, and you're doing a good thing for society! 