Learn Finnish: Juhlissa oli paljon nuorta väkeä. - There were a lot of young people at the party.

No! While the -kko / -kkö suffix often forms collective nouns, nuorikko specifically means a "young bride" or "newlywed wife." To talk about young people collectively, use nuorta väkeä (young folks) or the noun nuoriso (the youth).

Why is the verb oli in the singular when referring to a large group of people?

This is an existential sentence, which states that something exists in a location and typically starts with that location (Juhlissa). In this structure, the verb is always in the third-person singular (oli), regardless of how many people were there.

Why is the sentence using väkeä instead of ihmisiä (people)?

Väki (partitive väkeä) is a singular collective noun meaning "folks," "crowd," or "people." While paljon nuoria ihmisiä is perfectly correct, using nuorta väkeä is extremely natural and idiomatic in Finnish for describing the general crowd or demographic at a gathering.

Why is the adjective nuori in the singular partitive form (nuorta)?

Because the noun it modifies, väki, is grammatically singular. Even though it refers to a plural concept (people), it acts as a singular mass noun. Both the adjective and the noun must be in the singular partitive because they are expressing an unspecified amount following the quantifier paljon.