In general, use hyphens to aid clarity. Some examples:
'Abba will never re-form' does not mean the same thing as 'Abba will never reform.' The first sentence says the band will never play together again, and the second suggests that their past bad behaviour will never improve.
're-enter' is clearer on first reading than 'reenter' BUT cooperate and coordinator are fine without hyphens.
'It could be done by 10-year-olds' is clearer on first reading than 'It could be done by 10 year olds.'