I mean things like that take time, but the rate wi increase as well shure it will be a time of problems, but especially Japan has a shit load of completely unnecessary jobs (like the guys holding glowsticks when there is also blinking lights and barrier tape clearly marking a super tiny Building site) wich could just be removed without problems and many work places wouldn't need so many people if the old people would stop hindering digitalization and Progress... (thats kinda true for most first world countrys)
Your comment would be true IF old people were dying off and the age of people wasn't changing. You'd be replacing 80 year olds with babies. However, it's not so straightforward when you account for the fact that there are new people joining the 80 year old club every day.
Put very simply - if the birth rate dropped to zero today, would the population be getting younger since old people are dying off? Of course not, on the contrary the average age is getting older because there's more people joining the 80 year olds than the infants.