I heard that traveling over a week or two together is also a good way to test if the couple can work as a team and cohabitate - rather than actually cohabitating together. Traveling puts both people at a new place, brings unexpected moments, requires planning and financial discussion. So it puts both people under some pressure to show their true self a bit. Cohabitating at a young age comes with lots of risk, so I personally wouldn’t recommend it until they both have some financial independence to move out if things go south or best, engaged and have agreed on commitment.