The rhythmic pattern of the above line is iambic.
The line consists of ten syllables and follows an iambic meter, which means that it contains five iambs. An iamb is a metrical foot consisting of two syllables, with the first syllable unstressed and the second syllable stressed. In the line "When you are old and grey and full of sleep", the first syllable in each iamb (marked in bold) is unstressed, and the second syllable is stressed. This creates a pattern of da-DUM, da-DUM, da-DUM, da-DUM, da-DUM, which is the rhythm of iambic pentameter. Many famous poets, such as William Shakespeare and John Keats, have used iambic pentameter in their works.