We have taken many 4 year olds out on the trail - they usually have no problem managing the 1,5 hour trail and we have had 8 year old refuse to go anywhere near the horses.
So it depends alot on each individual, if they are confident and comfortable around animals, they will generally be absolutely fine on an outride.
This darling little girl, Angelique, is our youngest rider to date though - 2 years and 2 months. We had agreed on a quiet meander through the forest thinking that a short ride would be enough, but there was no turning back and she rode the whole way, forest trail and all!
Well done Angelique - we look forward to seeing you back soon!