Given that the only 22 year olds in either of the U23 races, or 18 year olds in the junior events, were those with January birthdays, I assume that the cut-off date for eligibility is 31st December, and there were no U23s in the elite race, Dutch or otherwise.
This will often throw those used to UK or Irish school sports, where from September to July, U16 (for example) means under 16 on the 31st August just before that competition year starts. So a kid with a birthday Sept to Dec could be U16 for autumn events in school sports, but already has been ineligible for 8 months or more according to organisations following an annual calendar, and from January, would be U16 in school, but not even U17 out of school.