1. We had a sleepover at Susan's house.

2. We had a sleepover party at Susan's house.

(Which expression is correct? Are both grammatical?)

Q2: Do only girls have a sleepover? How about boys? Do they have sleepover parties? Do all teenagers have sleepover party in the USA?

Both are correct, but 1. is more common.

Boys also have sleepovers. I don't know about ALL teenagers, but certainly most have sleepover parties.

1. Both expressions "We had a sleepover at Susan's house." and "We had a sleepover party at Susan's house." are correct and grammatical.

The choice between "sleepover" and "sleepover party" depends on personal preference and the context in which it is being used. In some cases, people may use the term "sleepover" to refer to a gathering where friends spend the night at someone's house, which may or may not involve additional activities or celebrations. On the other hand, "sleepover party" emphasizes that there are planned activities and festivities involved in the gathering.

2. Sleepovers are not limited to just girls; boys can also have sleepovers. Sleepover parties are not exclusive to one gender. Both boys and girls can have sleepover parties. Additionally, sleepovers and sleepover parties are not only limited to teenagers but can be enjoyed by people of all ages. In the USA, it is common for teenagers to have sleepover parties, but it is not a requirement for all teenagers. It depends on individual preferences and social norms.