When is the earliest you could expect a pullet to go broody?

One of my babies is acting very suspicious...

Yesterday I noticed that she went into the coop very early, but she has only just (within a couple of weeks) come into lay and her laying pattern is not set yet, so I thought she may just be laying late.

Today I come in and again she is sitting in the next box on 2 eggs which had been layed since I picked up the main lay this morning.  She was very reluctant to get up and had all her feathers fluffed up.  When I went to lift her out, she krr'd at me but let me lift her out without any problems.

Within 10 mins she was back on the nest (no eggs)

Does that sound broody?

She is only 20 weeks old.

she might be practising at being broody, I would doubt at that age she would old enough to be a mum, so wouldn't reccomend allowing her to sit Also at 20 weeks she should be putting her energy into maturing before the winter, so I would turf her off the nest and keep a close eye on her for laying problems.

depends on breed and when they were hatched, for example a late hatch silkie pullet may go broody very early spring, if not Christmas!