I think it depends on the child. I moved
my son out of his crib when he was 2 years
old, he was starting to crawl out of his
crib, and always wanted to sleep in my
bed, because it was a "big bed". So, I
went and got him his very own "big bed".
I do think 1 and a half and younger though
is too young, because you then risk more
falls, accidents, etc. You should make
sure you will hear your child wake, and
that your child will come to you or the
bathroom first. Be careful of stairs,
windows, etc.
Good luck.