Reader response criticism is a school of literary theory with focuses on the reader of a piece of work, rather than the author.
If applied to When You Are Old, the answer to the question 'What is the mood and tone of the poem' is reliant on the feelings/emotions that it arouses in the individual reader or 'audience.'
Providing you can justify your answer to the question, any opinion that you give is valid. My personal opinion is that it the mood is sad, and perhaps a little angry. The tone is cautionary - the' voice' in the poem is giving a warning to the person addressed that, because she has rejected the person who truly loves her, she may suffer a lonely old age.