I thought the rules were horses must be 4 and over for ridden competitions of all sorts (bar flat racing).
You can run a potential NH horse in bumpers as a three year old and they would probably already be jumping at home at that age. Personally as a NH racing person I wouldn't want to work anything until it was 4 although we are in a minority in that respect.
The 1st Jan only applies to TB's and that is the main reason why people breed as near to 1st Jan (but not before) that they can, this is mainly because they aim to sell as yearlings and want the most mature looking horse that they can, they also tend to wean at 6 months which I also don't approve of.
For all other competitions I thought it was no ridden competition before 4 and the date of 1st May was used?