The rules were different for officers and other ranks - officers could be sent to the front aged 18, other ranks 19. Obviously there were many who lied to get into the action during the early months of the war. These would turn up in France in 1915/16.
Regarding the lowering of the age for other ranks on 10 April 1918 as noted above, this was to some extent retrospective. I have just been researching a lad who was sent out in the opening days of April 1918 despite being under 19. The crisis of late March that caused the change didn't wait for the legislation to be passed, even though it was rushed through. In the case of
William Crocker it was seriously bad news - he died on 11 April, aged 18, after 9 days in France.