I think it can be easy to play the field as a young single, but most people eventually are looking for Mr or Ms Right and seem to become serial monogamists, or end up getting serious and settling down with one person.
However, I do know of some young people that attempt to practice ethical polyamory, but have a hard time with it, because one is still getting to know oneself in the 20's, and trying to learn how to be a good partner. So, doing relationships with two or more others can go very badly since they don't have their communication skills down yet, and in fact can be quite conflict-avoidant.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
In a newish relationship with Steve, (33, poly)
"Master," (mono, 36), miss pixi's Dom for 3 years