他们俩白头到老。用英语怎么说？

we go hand in hand forever.

The two of them grow old together. 他们一起变老
或
'Til death do they part. 他们到死都不分离

live with you in the rest of your life
出自老友记钱德乐对莫尼卡说的话~

They are always in love

他们白头到老：
They share the rest of their lives together.

I will live with you in the rest of your life