‘I don’t paint on walls every day; I need to have the nerve to do it. And the spray cans get heavy so it’s better if I have a friend along to help carry them,” says 65-year-old retired doctor turned street artist Luísa Cortesão.

In most cities, Cortesão would earn the title of the oldest graffiti artist in town, but not in Lisbon. Here, she is part of Lata65, an organisation that runs workshops for over-65s who are interested in making street art, at which the oldest participant today is 90-year-old Isaura Santos Costa. There are also several octogenarians among the group of eight women and three men, who have been dubbed the “graffiti grannies”, the youngest of whom is 59.

This Lisbon workshop is the latest in a series that Lata65 has run across Portugal and next month the organisation will implement its first international course – three workshops in São Paulo, Brazil, which the organisers hope will be the beginning of a larger international phase for the project.

Lata65 is the brainchild of architect Lara Seixo Rodrigues, 36. She teaches the workshops with street artist Adrião Resende, 33, and with help from Cortesão, who attended the first course and has been coming back ever since. Seixo Rodrigues began organising a street art festival in her home town of Covilha in 2011 and says it was then that she noticed that older people were particularly interested in the medium.

“Each year we ran the festival I noticed it was the older people, not the younger ones, who were really engaged,” she says. “They were our companions at all hours, day and night, asking us questions about how it was done and commenting on what the paintings represented. I realised there was a real interest in street art among this age group.”

Aida Alves, 76, masked and ready to paint. Photograph: Eduardo Leal

The two-day workshop begins with a short history of graffiti, covering everything from tagging (which elicits tuts of disapproval from some of the women) to the stencil-based work of British artist Banksy and others. There is even a tongue-in-cheek warning. “It’s best,” says Seixo Rodrigues, “to use a nickname when you write on the walls, just so the police don’t find you.”

The group titters a little anxiously but for this programme, at least, arrest is not something to worry about: the walls they will paint are supplied by Lisbon city council’s Galeria de Arte Urbana.

“Lisbon has become a global centre of street art,” says the department’s technical officer Claudia Silva. “The department was established as a platform to incentivise street art in the city by providing legal walls.”

After a day of designing and cutting stencils, the group meet at the designated wall on the second afternoon, Seixo Rodrigues carrying a big box of spray cans of every colour. There is nervousness as they pull on their face masks and rubber gloves but once the painting starts there is no stopping these artists. They jostle for space and complain when others paint over their favourite designs. Within half an hour, the pale pink wall has been transformed into a multi-coloured canvas, featuring everything from handwritten tags to stencilled horses, fish and flowers.

At one point 90-year-old Santos Costa turns from the wall in frustration – “I don’t have anything more to paint,” she says, asking Seixo Rodrigues to dig around in a pile of stencils to find her another piece. Aida Alves, 76, keeps one arm against the wall, protecting her space as she takes a break. “It’s great fun,” she says, and Olinda Rodrigues, 66, standing beside her, agrees.

“The more I paint, the more I want to paint,” she says. “I didn’t really like street art that much before this; I always thought it was just kids making a mess of the walls. But now I understand the history behind it and the way of thinking and I appreciate the artists more.”

At 59, Maria Manuela Graça is the youngest of the group. She is already forming plans for the future. “I’ll definitely do it again. There’s a wall in my neighbourhood that’d be great to paint on.’