To ask "How old is the song LOVE IS TEASING?" is a lot like asking, "How old is the car that Johnny Cash built, according to the song ONE PIECE AT A TIME?" (Read the lyrics to that song and you will understand the problem.)

We could consider the song line by line.

How old is the line "Love is teasing"? Well, I can find it in a song called LOVE'S A TYRANT that goes back at least to 1831:

And ah! 'tis useless to complain:
Though love is teasing,
'Tis also pleasing.
And pleasure yields as well as pain.

But I can't verify the line "love's a pleasure when first it's new" any further back than 1975; but the variant "love is a treasure when first it's new" goes back at least to 1957.

However, in a slightly different form, it appeared in WALY, WALY, GIN LOVE BE BONNY back in 1714:

O waly, waly, but love be bonny,
A little time while it is new,
But when 'tis auld it waxeth cauld,
And fades away like the morning dew.

I could keep going, but I don't think I could ever give you a simple answer.