We are just about, or not quite, 30 and will not pay attention to this issue. Long before a staircase should become an obstacle, for example, all the garden work, house cleaning, and everything else around it simply becomes overwhelming. Also, in a two-story house, in my opinion, it would be a great waste to limit oneself to just one floor. Besides, we are planning a study on the ground floor, which could at least serve as a single bedroom, and a shower on the ground floor is also planned.
However, we are building in a very good location, i.e., close to the city center, and would probably be able to sell the house easily and quickly for a good price. Or inherit it earlier. In any case, I do not rule out that maybe at 50, 60, or 70 years old we simply move back to a city apartment without garden work, without much cleaning, and other duties like snow shoveling. For the next 20 years, a house is certainly optimal, but after that, one can switch again, or in 30 or 40 years.
In America, the trend to change houses quickly has existed for a long time, and such a trend is known to spread here after at most 10 years. We also know people who move from a family house to a newly built bungalow after the children have moved out. They are about in their mid-50s. And then perhaps, in their mid-70s, back to the city and sell the bungalow? Other acquaintances want to move from the family house back to a city apartment now in their late 50s, which we also do not exclude.
To put it into perspective: Those who have built in the countryside and will probably only get much less than the value for their house and who perhaps do not like a city apartment anyway, should maybe pay more attention to this. Our nonchalance towards this issue also partly stems from the fact that building land and family houses are extremely in demand here and will remain so in this region.