Horses, like people, reach sexual maturity at different times. While 2-5 may be the average time frame, there will always be the examples that will be above and below it. It is possible that a yearling could begin cycling, but it's not something terribly common.
If there is a stallion nearby it could cause her to come in sooner, I noticed with some of the young fillies around that would act like they were in heat and bug the stallions but they were not actually dropping an egg, it is more like a submissive thing they do.
Just make sure that from about 9 months on, she's never with an intact stallion or you'll have a 2 y.o. Dropping a foal. BTDT, got lucky, she was an awesome mother and the foal is lovely. Actually, my fav of 3 born that year.