It was sang by George Formby Senior, father of the GF that played the uke.
It has three verses and the chorus is:
I'm twenty-one today, twenty-one today,
I've got the key of the door, never been twenty-one before
and Pa says I can do as I like, Hip hip hip hooray,
He's a jolly good fellow, I'm twenty one today
The first verse I give what I remember and ... for gaps
I am all done with college life now I'm one of the boys
2nd verse ...Got excited and smacked a policeman's face!
I will be back as I remember more