This is backed up by Terence Smith's 1984 NYT article:

THEIR TICKET TO PARADISE IS the blood-red headband and the small metal key that they wear into battle. "Sar Allah," ("Warriors of God"), some of the headbands read in Farsi script, identifying the wearers as divinely designated martyrs who will use their keys to go directly to heaven if killed in the holy war against Iraq declared by their leader, the Ayatollah Ruhollah Khomei ni. The headbands and the keys are worn by young boys, aged 12 to 17, who are recruited by local clergy or simply rounded up in the villages of Iran, given an intensive indoctrination in the Shiite tradition of martyrdom, and then sent weaponless into battle against Iraqi armor. Often bound together in groups of 20 by ropes to prevent the fainthearted from deserting, they hurl themselves on barbed wire or march into Iraqi mine fields in the face of withering machine-gun fire to clear the way for Iranian tanks.

The article continues, specifying the source of the information:

In dozens of interviews conducted by this reporter in recent weeks with Iranian exiles, academics and government and intelligence officials in the United States and Europe, the blind faith of these teen-age martyrs was frequently cited as symbolic of the fanaticism that is part of life today in the Islamic Republic of Iran. An East European journalist who witnessed one of these human-wave assaults, in which tens of thousands of young Iranians have gone willingly to their deaths, could hardly believe what he was seeing, as first one boy, and then another, detonated a mine and was hurled into the air by the explosion. "We have so few tanks," an Iranian officer explained to the journalist, without apology.

I found similar information on Refworld citing "The Abuse of Human Rights in Iran, London: House of Commons, Parliamentary Human Rights Groups, 1986, p.41." but I couldn't find that to follow up. It says they used "[b]oys as young as nine".

This is backed up by Terence Smith's 1984 NYT article:

THEIR TICKET TO PARADISE IS the blood-red headband and the small metal key that they wear into battle. "Sar Allah," ("Warriors of God"), some of the headbands read in Farsi script, identifying the wearers as divinely designated martyrs who will use their keys to go directly to heaven if killed in the holy war against Iraq declared by their leader, the Ayatollah Ruhollah Khomeini. The headbands and the keys are worn by young boys, aged 12 to 17, who are recruited by local clergy or simply rounded up in the villages of Iran, given an intensive indoctrination in the Shiite tradition of martyrdom, and then sent weaponless into battle against Iraqi armor. Often bound together in groups of 20 by ropes to prevent the fainthearted from deserting, they hurl themselves on barbed wire or march into Iraqi mine fields in the face of withering machine-gun fire to clear the way for Iranian tanks.

The article continues, specifying the source of the information:

In dozens of interviews conducted by this reporter in recent weeks with Iranian exiles, academics and government and intelligence officials in the United States and Europe, the blind faith of these teen-age martyrs was frequently cited as symbolic of the fanaticism that is part of life today in the Islamic Republic of Iran. An East European journalist who witnessed one of these human-wave assaults, in which tens of thousands of young Iranians have gone willingly to their deaths, could hardly believe what he was seeing, as first one boy, and then another, detonated a mine and was hurled into the air by the explosion. "We have so few tanks," an Iranian officer explained to the journalist, without apology.

I found similar information on Refworld citing "The Abuse of Human Rights in Iran, London: House of Commons, Parliamentary Human Rights Groups, 1986, p.41." but I couldn't find that to follow up. It says they used "[b]oys as young as nine".