And they'll move out of the cities into suburbia in their 30s, as they always do.
https://www.bbc.co.uk/news/uk-47529562
Over the past decade, about 550,000 more Britons left London than moved to it.
This trend is not unique to London; it is seen in many big US cities, including New York, Miami, Chicago and Los Angeles.
...
People arrive in their 20s looking for jobs, then retreat in their 30s. Although it seems more likely they they only retreat to "commuter" distance -- in London that's about 40-60 minutes from a main rail terminal, which of course leads to increased pollution.
...
The vibrancy and job opportunities offered by our biggest cities appeal to students and young professionals. This shapes these cities in terms of the shops, bars and other facilities they offer.
But among older residents, the desire for things like more space and a less urban environment grows, and many move out.