The agencies presented a report in Rome on Friday that said more than 70,000 unaccompanied minors reached Italy between 2014 and 2018, many of them rescued at sea from Libya-based human traffickers' unseaworthy boats.
Some 90 percent were 15-17 years old and now are adults. UNICEF official Anna Riati said the difference is "negligible" between a refugee or migrant aged 17 and one aged 18 when both have lived through similar traumatic experiences.
The report warned that loss of protection afforded to minors can put young adults at risk of sexual exploitation and other abuse.
A top European human rights official says police and border officers in the Balkans must refrain from pushing back migrants crossing into their countries while trying to reach Western Europe.
Council of Europe Commissioner for Human Rights Dunja Mijatovic on Friday called for "efficient and independent" investigations into allegations of ill-treatment of migrants.
Mijatovic issued the statement after speaking to Bosnian Minister of Security Dragan Mektic.
She also welcomed the announcement by Bosnian authorities that they would close down a tent camp in northwestern Bosnia that has faced international criticism because it is located on a former landfill and near a mine field from the 1992-95 war.
Hundreds of migrants are staying in the Vucjak camp near Bosnia's border with Croatia. Mijatovic says their relocation is urgent because of approaching winter.
Bosnian police say they have stopped a van carrying 17 migrants and captured two suspected people smugglers near the border with Croatia.
Police said Friday the migrants told the officers they were from Iraq and Syria and had no documents. They were stopped Thursday near the southwestern town of Ljubuski.
Police say the van driver and another man, who was in a separate car, are facing charges of people smuggling. Both men are Bosnian citizens and one also holds Croatian citizenship.
Thousands of migrants are stranded in Bosnia looking for ways to cross into neighboring European Union member Croatia. People fleeing war and poverty in their countries seek to reach Western Europe through the Balkans.
Police in Slovenia say a 20-year-old refugee from Syria has died after family members rescued him from a forest in the south of the country.
Police said Friday that the man died Thursday in the southern town of Ilirska Bistrica from hypothermia and exhaustion despite being given medical assistance.
They say the man called cousins who live as refugees in Germany to come find him in the forest, gave the coordinates of his position and said he was unable to continue.
After finding him, the cousins took the man to a nearby police station but doctors couldn't help him, the STA news agency reported.
The incident illustrates the dangers migrants face while trying to reach wealthy nations of the European Union.
A leading international human rights body says Croatia should not be allowed into the European Union's border-free travel zone because of its treatment of migrants crossing into the country from neighboring Bosnia.
In a report Friday, Human Rights Watch criticized the EU's executive Commission for saying last month that Croatia is ready to join the so-called Schengen Area.
It said Croatia's "unlawful and violent summary returns of asylum seekers and migrants should disqualify it from joining the Schengen Area" and that the Commission's actions "sends the message that serious human rights abuses are no obstacle to Schengen accession."
There was no immediate reaction from Croatia to the report. Croatian officials have repeatedly denied accusations of abuse from migrants and human rights groups.