Accused robber Martin Chatfield had been an adult for just a few days when he allegedly stole $300 from a Weston restaurant at scissor-point.
But police say his accomplice remains at large.
Chatfield, who turned 18 last week, appeared before the ACT Magistrates Court on Wednesday charged with aggravated robbery.
Police said two men entered Subway in Weston about 9.20pm on Tuesday.
The pair allegedly covered their faces with their shirts and used a pair of scissors to threaten staff and demand cash.
The men then fled the store and were confronted by police.
Chatfield was apprehended a short time later and police are on the hunt for a second man.
The defence said Chatfield's family and friends had serious concerns about his mental health.
Magistrate Beth Campbell marked the Wanniassa man as a prisoner at risk and ordered a mental health assessment be carried out if required.
Chatfield will reappear next week.
Police said the co-offender fled the scene in a blue station wagon, with tinted windows, in the direction of Namatjira Drive.
The vehicle is suspected to be stolen.
The man is described as Aboriginal in appearance, about 170 centimetres tall with a slim build.
Police called for witnesses to the incident or anyone who saw a car matching the description to contact Crime Stoppers on 1800 333 000 or via www.act.crimestoppers.com.au.