HP washing questions

so ive been reading, in passing, a couple of threads on hp washing. so i was wondering,
1) can you hp wash at low levels like level 20? and im guessing you would be handicapped, because you dont gain alot of hp in first job? please elaborate
2) is there a minimum level that you should start doing it, ive seen people say for ex. level 120 or level 90? does it matter? for ex. if you start at level 120 would you be handicapped in some way because you didnt start at level 90?
3) the previous question leads to my 3rd question, cant you just hp wash everything at the end, like level 200? please elaborate i.e. would that be possible? if yes/no, explain

i apologize if these questions have been asked before