I bought this for my son for when we go kayaking.  He is 4 years old and I purchased the medium because another reviewer stated that the large was for children over 7.  I would recommend purchasing the Large.  There is a toggle on the back of the hat to adjust the size.  So you can adjust the size as your child grows.  The medium should do fine for this year and next if I am lucky.  But if I had to do it again, I would go with the large.  I purchased the glacier/dark grey.  It is a very pretty dark bright blue.  I should have no problems spotting him in a crowd.  Also, there is a safety feature that I like.  You can tighten the hat under the chin so your child will not lose the hat in a wind, but there is an extra connector that will release the hat if it get snagged and becomes a choking hazard.  I highly recommend.