Cahill is among a 23-strong squad to open the Socceroos' World Cup qualifying campaign in Kyrgyzstan on 16 June.
Postecoglou has leant on familiar faces for the fresh World Cup campaign, with 19 of his Asian Cup winners, and no uncapped player, in the squad.
Cahill turns 36 this year and would join elite company if he plays at a fourth World Cup finals, including Pele and Maradona.
Only German Lothar Matthaus and Mexican Antonio Carbajal have played at five World Cups.
Cahill, Australia's all-time leading goalscorer, remains a linchpin in Postecoglou's plans.
"His long-term position is that he is still getting selected for the team," Postecoglou told reporters in Sydney on Monday.
"The last time he represented his country he helped us win an Asian Cup.
"I'm not going to put pins in where people end their careers or begin their careers.
"Whether they're 18 or they're 35, if they fit into what we're doing and they fit into the way we want to play our football and what our culture is then they'll get selected - and Timmy is in that at the moment."
Postecoglou said the naming of the squad for the Kyrgyzstan mission should end any lingering Australian satisfaction at winning the Asian Cup in January.
"It maybe puts a little bit of a full stop to the great after-glow of the Asia Cup, we can start looking forward now," he said.
"This process is about qualifying, which is always a tricky affair and will be challenging for us even in this first phase ... not just qualifying but building a really strong team over the next two or three-year cycle.
"This squad is pretty reflective of our journey over the past 18 months, the World Cup and the Asia Cup squads.
"And we are beginning to form a little bit of a foundation of the team we want to be and the players we're selecting."
But Postecoglou stressed the settled squad shouldn't be mistaken as a sign of being comfortable with Australia's stocks.
"I'm never comfortable, mate, and neither are these players," he said.
"They get the first opportunity to nail down a spot but nobody will be comfortable."
The Socceroos will meet Kyrgyzstan, Jordan, Tajikistan and Bangladesh in home and away fixtures to decide the initial qualifying phase for the 2018 World Cup finals in Russia.
