President Recep Tayyip Erdogan said after Saturday’s attack in Gazantiep that the bomber was 12-14 years old.
Turkey has linked the bomber to so-called Islamic State (IS), but Mr Yildirim said “a clue has not yet been found concerning the perpetrator”.
His statement came as Turkey’s military targeted IS militants in Syria.
Television reports said howitzers had been used against IS near the border town of Jarablus.
Turkish artillery had also hit US-backed Syrian Kurdish YPG positions north of Manbij in Syria, broadcaster NTV said.
A coalition including YPG has been pushing IS out of Syrian towns, including Manbij, recently.
Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), a Turkish-Kurdish rebel group fighting for autonomy since the 1980s.
Speaking to reporters after a cabinet meeting on Monday, Mr Yildirim said the earlier statement identifying the attacker as a child was a “guess” based on witness accounts.
Most of the victims were children, media reports say.
Twenty-nine victims of the attack, which took place on Sunday, were under the age of 18, reports said, with one official saying 22 were under the age of 14.
Thirteen of those killed were women, Turkish media said. Sixty-six people are still in hospital, 14 of them in a serious condition, Dogan news agency reported.
One woman lost four children in the attack, the Haberturk newspaper reported. Emine Arhan told the title “if it wasn’t for my only surviving child, I would have killed myself”.
Another victim was a nine-year-old girl who had stayed on at the party to see the bride after her parents had left, according to the Vatan newspaper.
A disproportionately large number of women and children were killed in the attack because it targeted henna night, a part of the celebration attended mainly by women and children, says BBC Monitoring’s Turkey analyst Pinar Sevinclidir.
Hurriyet newspaper said the type of bomb, which contained scraps of metal, was similar to those used in previous attacks on pro-Kurdish gatherings.
Prosecutors said a search was also under way for two people believed to have accompanied the suspected attacker to the wedding party but who left before the blast.
Gaziantep, near the Syrian border, is known to contain several IS cells.
In a defiant speech on Monday, Turkish Foreign Minister Mevlut Cavusoglu said IS should be “completely cleansed” from the border area with Turkey.
He also announced that Turkey was recalling its ambassador to Vienna because of a demonstration in the Austrian capital by groups associated with the PKK.
Mr Cavusoglu accused Austria of “supporting a terrorist organisation which is attacking Turkey”, according to Austrian paper Der Standard.
Ties between the two countries have been fraught over the past weeks, with Vienna warning that Turkey is heading towards authoritarian rule, and calling for its membership talks with the European Union to be ended.