Gedhun Choekyi Nyima was declared by the Nobel laureate as the reincarnation of the Panchen Lama, the second most senior figure in the Gelug school of Tibetan Buddhism, at the age of six in 1995.
The boy was then taken into custody by Chinese authorities and has not been seen since, with Tibetan authorities in exile describing him as "the world's youngest political prisoner".
The Dalai Lama's designation of him was "illegal and invalid", he said.
Beijing appointed Gyaincain Norbu as its own 11th Panchen Lama, who is now a deputy to the Chinese People's Political Consultative Conference, a national debating chamber, but many Tibetans do not recognise him.
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He has made numerous tightly scripted public appearances since he turned 18, and visited Hong Kong, his first trip outside the Chinese mainland, in 2012.
Asked whether Nyima was still alive, a Chinese foreign ministry spokesman declined to respond explicitly today.
"What I know is consistent with what the Tibetan official has briefed you about," Hong Lei told reporters at a regular briefing.
The Central Tibetan Administration, the government-in-exile, lambasted Beijing for detaining Nyima.
"As long as people are not given free access to meet him, we will consider him as missing and held against his will," Dicki Chhoyang, minister for information and international relations, told AFP.
The officially atheist Communist Party reiterated its right to control the process of reincarnation in a white paper issued at the weekend to mark the 50th anniversary of the forming of the Tibet Autonomous Region, China's administrative designation for the area.