When a baby is born there digestive system is not very developed and is not developed enough to eat any solid foods.
But when the baby gets to 6 months of age the digestive system is developed enough to eat solid foods so when you're 6 months of age your digestive system has been fully developed.
After 6 months of age your digestive system is still likely developing and usually continues to develop until you reach at least age 18.
Then by age 18 years old your digestive system is fully and entirely grown and developed and will no longer develop.