This one stole the whole piece for me!

When you're getting old -
I'll be your supporting stick,
your third walking leg

It reminded of a riddle about man. 
What goes on four legs in the morning, on two legs at noon, and on three legs in the evening?

The Senryu is not used in that sense, but I think is beautiful how love will help them support each other that way. Becoming the thrid leg. The cane per se.