They're absolutely everywhere and have become a major battleground for parents and children in the home.
Smartphones have been linked to a decline in mental health, sleep disturbance and screen addiction in numerous studies, but life as we know it wouldn't function without them.
One city in the United Kingdom is attempting to keep the devices out of young hands until they're at least 14.
Multiple parents from schools in St Albans, south-east England, have signed a pledge not to give their child a smartphone before that age.
One of those parents Jessica Pyne says it's collective action she and others had been desperately waiting for.
"Nobody wants their child to be the odd one out, everybody wants to fit in particularly at that tender age, so if everybody is holding off until 14, then the expectation has shifted," Ms Pyne said.
"If we're all feeling this way, then actually let's all stick together and if nobody has a mobile phone when they're 11, and the expectation isn't there, then we can hold strong."
The campaign has been running for six months and makes a distinction between smartphones and simple phones, which can be used for communication but don't connect to the internet.
Executive head teacher at Cunningham Hill Schools in St Albans Justine Elbourne-Cloud says transitioning kids onto simple phones is a key objective of the smartphone-free childhood movement.
"We do talk about the alternatives to smartphones, the dumb phone or the brick phone, the old Nokias that we used to have," Ms Elbourne-Cloud said.
"Parents want to know that they can contact them or that they can phone you and say they missed the bus or whatever...especially as the phone box is a bit of a thing of the past, but it doesn't have to be a smartphone."
According to a survey of 2,480 parents by Ofcom, the independent UK regulator for communications, around a quarter of five to seven-year-olds in the UK own a smart phone and that number jumps to 95 per cent for 12 to 15-year-olds.
Ms Elbourne-Cloud says increased access to smartphones outside of school is impacting student performance in the classroom.
"Over the last few years, what we've really noticed is the attention span of our children getting less and less," she said.
"It's now called the 'TikTok brain', the idea that children have such a short attention span that trying to be in a classroom teaching everything has to be quick-fired and moving on because they can't swipe teachers, they can't swipe the classroom, but they have to stay focused."
St Albans' ambitious plan to be the first city to implement a smartphone-free policy for children under 14 is gaining momentum in other parts of the country too, with the parents of more than 66,000 students signing the pledge from the grassroots not-for-profit group Smartphone Free Childhood.
But speaking to 12-year-olds at the City of London Academy high school, the idea was quickly shut down.
All of them already had smartphones and had no plans to give them up.
"It would be quite challenging because I'm used to my phone already and I use it quite a lot as well," Year 8 student Roman Kandelaki said.
"Without my phone, I feel different, I don't feel the same, I feel like I'm a different person."
His classmate Nosa Aiwuyo agreed.
"I think I'd miss out on stuff because there's lots of stuff that's happening on social media that people miss out on when they don't have phones," he said. Twelve-year-old Nahla Nadji said she would also struggle to give up her smartphone despite acknowledging downsides to owning the device.
"The best thing is you can really do anything on them, you could take pictures, videos and everything but the worst thing is it's how people get bullied the most," she said.
Phones can be 'tool of learning'
The British government is considering a new law to ban smartphones in all schools in England. It's a policy France is currently trialling.
Australia, and some US states, are also exploring social media bans for children amid growing claims its use is linked to a decline in mental health.
Psychologist Dr Jonathan Haidt is an official ambassador of the Smartphone Free Childhood campaign.
His bestselling book The Anxious Generation has struck a chord with many parents and links a surge in anxiety, depression and self-harm in young people with the explosion of smartphones and social media since 2010.
But some experts, including Vojtech Mýlek from the Interdisciplinary Research Team on Internet and Society at Masaryk University in the Czech Republic, argue the evidence isn't as straightforward or conclusive as what's being presented.
"If I give the example of social media use and correlations with depression or depressive moods, when we see this correlation, we can conclude that social media leads to more depression or we have the same amount of evidence for the other way, maybe more depression leads to more social media use," Mr Mýlek said.
"Every time you see correlational evidence, you have to assume that both of the directions of causality are possible."
Professor Sonia Livingstone from London School of Economics' Digital Futures for Children says there is evidence that suggests restricting phone use in schools is beneficial, but she believes there are alternatives to outright bans.
"Children don't always live in perfect situations, so the phone can be a way of escape or help from difficulties in the offline world," Professor Livingstone said.
"It's also a tool of learning ... and we are almost at the point, I think, of sacrificing all of those benefits because we are so fearful of the risks that we haven't managed to address in other ways."
One thing experts and campaigners agree on is the need for tech companies and social media giants to step up regulation of their services.
"I think the movement for a Smartphone Free Childhood, and lots of the other bans that are being called for, is movement born of frustration," Professor Livingstone said.
"A lot of parents have wanted governments to regulate big tech for a long time and it isn't happening and they feel now they have to take action into their own hands."
