Henry III was born in 1207 and became King in 1216, at the age of nine. His father was King John, brother of King Richard the Lionheart and famous for having unintentionally triggered the introduction of the Magna Carta in 1215. It is again improbable that Henry had any power during his youth and he had several regents including the popular William Marshal, who ruled the country.

The main problem for Henry was that his regents and the nobles naturally upheld the statutes of the Magna Carta during his minority and this gave the charter ten years to sink in to the normal workings of government. This probably was what made life more difficult for the King throughout the rest of his reign, even if it benefited English law in the long term.

It seems that Henry spent a lot of his reign trying to fight against and undo the dramatic changes that had been forced on the monarchy during the reign of John. He hated and distrusted the barons, wouldn’t take advice from them and surrounded himself with foreigners, who, due to their being more autocratic systems of government in Europe, may have been there to remind Henry of his royal supremacy.

The impact of the Magna Carta is debatable and some argue that Henry’s view of his power had not changed because of it:

None of this suggests that Henry was weak, or that Angevin kinship was breaking down. On the contrary, Henry was acting like his father and grandfather and exercising full authority over all who governed in his name. Henry’s weakness was in his incapacity to direct this centralised system of government along the right lines.

Contrary to common belief the baronial opposition to Henry III was not directed against a weak king who was unable to rule, but against a king who was determined to rule without consulting the men who regarded themselves as his natural counsellors.

From The Community of the Realm in Thirteenth Century England by Roger Wickson

It is possible to argue that, had Henry come to the throne in his majority, the barons would not have had enough time to fully instigate and rule the country by the laws of the Magna Carta. Surely Henry would have taken action to prevent them from gaining too much power? Perhaps then, Henry’s coming to the throne during childhood didn’t do him much good in the long term; it didn’t give him much of a chance to fight back during the crucial first years of his reign?
Its an interesting question – probably demanding a lot more research ( =