Introduction: Abnormal body weight, including overweight and obesity, is a common health problem affecting children and adolescents. The present study aimed to analyse weight changes in children from preschool age to adolescence and to identify early predictors of excessive weight in adolescence, such as blood pressure and physical fitness observed in preschool children. Methodology: Data from 3075 children (1524 girls and 1594 boys), collected as part of the Gdańsk Centre for Health Promotion’s “Your Child’s Healthy Life” programme, were analysed, with each child assessed at ages 6, 10, and 14. Results: The results indicated that boys were more likely to be overweight, with a tendency for obesity to increase with age. Children who were overweight or obese at age 6 had a higher risk of remaining so for a longer period of time. In addition, low physical fitness (as measured by the KPRT test) and elevated blood pressure were significantly associated with excess body weight. Conclusions: The study underscores the importance of early intervention and consistent monitoring of childhood overweight and obesity to reduce their long-term impact on health.