The only "allowed" solution is importing people from poor countries into richer ones, but disguised as something humanitarian, taking in refugees of war or whatnot.
Eventually, likely in the direction of gun control - birth control being licensed or the like. Or being religious being a requirement again to be a ‘respected member of society’.
Unlikely to be popular of course, but eventually society will change to protect its interests - or die.
I wish we could have real conversations about how to solve this issue, and not bury our heads in the sand. I'm not proposing any particular solution, just for our collective attention to the problem and liberty to discuss solutions.
People seem to think "oh, I'm paying my social contributions, that should be enough for my retirement", but the whole formula really doesn't factor in a sharp drop in workforce we're about to experience. We're papering over the monetary deficit with credit all over the world, but that's not feasible long-term, and the "who's actually gonna do it" aspect will become much more important. Writing a check doesn't change the senior's diaper, nor anything else.
As for practical solutions, first you have to make it possible for young people to obtain real estate again somehow (crucial), then to subsidize kindergartens / child care of all kinds heavily, and only then go more radical.
Another radical angle could be some sort of education reform where people are somehow incentivized to do all the post-BSc education in their 30s instead of 20s. How to achieve that without horrible side effects is up to debate. Subsidies for parents on every corner and every aspect of life (including education) would probably work, but how much is the optimum and how to implement it is a serious question.