realistically we will likely have a mission to proxima, uhh .. in the next 100 years?
i see no problem in grooming a team of 20yo astronauts for a 20year journey, and it's likely this is something WE WILL REALLY WANT TO DO. And we'll do it. And it will be useless. I'm purrrty sure we had a moonbase planned for the 80s during the 60s.
I strongly doubt we will "expand", as in having a second earth on Mars, our ideas of space colonization exist because we are still very primitive psychologically.
Space exploration is futile. If you assign a value to how advanced a race is, you'll see that at our stage growth in population is no longer a contributing factor.
Right now, we want to explore space for two reasons, one is WOW SPACE EXPLORATION, and another is a concern for our continued existence as a race. When we will be more mature and we'll have a more grounded idea of what evolution means, we won't be looking to go into space. We CERTAINLY will not be "moving our race ot other planets", because you ain't moving a few billion people off world.
(..this completely hypotetical of mine would easily solve Fermi's paradox, by the way..)
Also consider that eventual colonization will likely *not* have the effect we imagine.
TLDR: we are too primitive to make plans.