Earlier quoted context omitted.
A Mars colony is something humanity should start thinking seriously about once we successfully run a production-grade lunar colony for about a decade-- long enough to recognize and tackle the worst bugs.
We can make sustainable space habitats now. They solve the same problems a Mars colony do, for a fraction of cost, with no huge rockets, months-long trips, and certain death if something goes wrong.
Except having a solid ground under your feet.
We haven't really used spin gravity in practice yet, so there are technical problems to solve there.