Right around 18 months children typically move from parallel play into associative play. During associative play, children still play primarily independently of each other, but they are more likely to engage in the same type of play. Children will imitate each other and play the same activity with a little bit of interaction with each other around 18 months. You may notice that your child will watch another child, then if that child is playing with a play kitchen, your child may pick up a pot and play beside that child, using the same toys. This is usually led by an older child. Keep providing opportunities for your child to play with other children to build his skills of social interaction.
During independent play, you will probably notice that your 18 month old is still engaging in symbolic play, playing with objects that represent real things, such as dolls, matchbox cars and play kitchens. You will start to see that they are sequencing activities a little bit more. For example, pouring pretend pasta in the pot before stirring it or feeding the doll before covering her with a blanket.
Resources:
Zero to Three
The American Academy of Pediatrics