Can i say "when i'll be older,i'll buy a motorbike"?
or better is "When i am older, i will buy a motorbike".
What is the general rule using WHEN in future sentences?
The general idea, Tense, and it's not a rule because we can sometime use 'will' in these situations, is that 'will' is predictive but this situation isn't predictive. The assumption is that you're going to live and it not an if or or, you will get older.