I only have experience with female ducks, but they can maintain a close bond after they've grown up.
note: ignore the fact that some of our ducks have masculine names - they were named before their sex was known and the names and he/she pronouns stuck...
Victor and Ming Mei were raised separately from the other ducks. They always stick together and if one goes wandering off, the other always follows. For a while Ming Mei would quack loudly and angrily whenever we separated her from Victor. She still doesn't like being separated but she learned not to freak out so much... They're both around 1.5 years old now. When one of them gets broody, the other acts as a sentry and tries to scare me off when i come in the room so I won't steal their eggs. lol
Vinny and Ollie were raised together by someone else so they also bonded to each other very closely. They also follow each other every where and quack for each other when separated. When Victor, Ming Mei, Vinny, and Ollie are all in the bathtub together, Victor and Ming Mei always stay on one side and Vinny and Ollie on the other.
The youngest ducks (2 months) are Moxy and Norie. Moxy just recently started quacking sadly when she can't see Norie. Norie on the other hand is the most codependent duck ever! The second Moxy is out of her sight she calls for her and is inconsolable until she sees Moxy again. We had to separate them for a few days because Moxy was sick and Norie was very upset the entire time.