The Myitkyina Township Court also sentenced two minors to 10 years, according to a senior police officer who spoke on condition of anonymity.
They were all arrested during a military crackdown on illegal logging and timber trade in January.
They had already been sentenced in late April to six months in prison for entering the country illegally.
Myanmar banned the export of teak logs for the first time in April to reduce deforestation, but illegal logging is still rampant in remote border areas, especially in northern parts of the country.
The country's forest cover fell by about one-fifth between 1990 and 2010, from 58 per cent of total area to 47 per cent, according to government figures.