Seid
Wie alt seid ihr?
How old are you (plural)?
Sind is used when the subject is plural, wir (we) or Sie (formal you)
Example:
Wir sind... = We are...
Die Jungen sind... = The boys are...
Bist is used for du (informal you)
Example:
Du bist... = You (informal) are...
Ist is used when the subject is singular
Example:
Der Vater ist... = The father is...
Seid is used for ihr (plural you)
Example:
Wie alt seid ihr? = How old are you?
