ISTANBUL — A child suicide bomber killed at least 51 people and wounded nearly 70 others at a Kurdish wedding party near Turkey’s border with Syria, President Recep Tayyip Erdogan said Sunday, decrying the attack as an attempt by Islamic State extremists to destabilize the nation by exploiting ethnic and religious tensions. The bombing late Saturday in Gaziantep was the deadliest attack in Turkey this year. It comes amid ongoing struggles between the government and Kurdish militants linked to the outlawed Kurdistan Workers’ Party, known as the PKK, and as the country is still reeling from the aftermath of last month’s failed coup attempt, which the government has blamed on U.S.-based Muslim cleric Fethullah Gulen and his followers. Erdogan said immediately after the Gaziantep attack, which he blamed on IS, that any strategy “meant to incite the citizens against each other along ethnic and religious lines will not work.” Later, addressing the nation before Istanbul’s city hall, Erdogan said the attacker in Gaziantep was aged between 12 and 14.