The present study examined the prevalence of behaviours in dogs separated from the litter for adoption at different ages. Seventy adult dogs separated from their dam and littermates and adopted between the ages of 30 and 40 days were compared with 70 adult dogs that had been taken from the litter for adoption at two months. Owners were asked to complete a questionnaire eliciting information on whether their dog exhibited potentially problematic behaviours when in its usual environment. Binary logistic regression analysis was performed to investigate whether the age at which the dog was separated from the litter might predispose it to developing undesirable behaviours. The odds of displaying destructiveness, excessive barking, fearfulness on walks, reactivity to noises, toy possessiveness, food possessiveness and attention‐seeking were significantly greater for the dogs that had been removed from the litter earlier during the socialisation period. In addition, dogs purchased from a pet shop at 30 to 40 days of age were reported to exhibit some of the listed behaviours with a significantly higher frequency than dogs purchased from a pet shop at two months. No significant differences were observed with dogs obtained from other types of sources. The dogs in the youngest age group (18 to 36 months) had a higher probability of displaying destructiveness and tail chasing. These findings indicate that, compared with dogs that remained with their social group for 60 days, dogs that had been separated from the litter earlier were more likely to exhibit potentially problematic behaviours, especially if they came from a pet shop.