The Nurse makes the effort to recall exactly how old Juliet is through a series of unfortunate events. She recalls her own daughter which likely gave her the ability to be a wet nurse for the Capulets, and the her husband used to laugh at Juliet falling as a young walking girl. She recalls weaning Juliet off breast-milk, and that produces a series of laughs.
Lady Capulet simply recalls her age.
I think this demonstrates that the Nurse has a fond relationship with Juliet. Nostalgic memories capture her mind when thinking fondly over the years. For Lady Capulet, this is a matter of business. At a certain age she feels she needs to tell her daughter to do specific tasks. This accentuates the division between mother and daughter.
With the nurse I think there is an under-current of hostility or bitterness that leads the nurse to help Juliet further the romance with Romeo and what would be contrary to the Capulet's wishes. Having to nurse Juliet rather than her own daughter must have been painful. She seems to delight in Juliet's discomfort and pain. And I find it somewhat uncomfortable that she refers to the younger Juliet as "it" no less than 8 times in her opening lines. Something to think about anyway.