But since then, she's de-transitioned, after realising "the vision I had as a teenager of becoming male was strictly a fantasy and that it was not possible".
"I was being perceived as a man by society, but it was not enough," she added.
"I felt like a fraud and I began to feel more lost, isolated and confused than I did when I was pre-transition."
Now 23, Bell is suing the government for allowing her to undergo the radical therapy, which she fears may have damaged her ability to have children.
She argued that underage children cannot truly understand what they're signing up for when they go through the life-changing therapy.
Bell thought she wanted to be a man after "a highly traumatic childhood".
She also suffered from body dysmorphia since she was 4 years old, not fitting into gender stereotypes and feeling disgusted by her body.
The three judges in Bell v Tavistock ruled that children under the age of 16 wouldn't be able to properly grasp the consequences of consenting to using puberty blockers.
"It is highly unlikely that a child aged 13 or under would be competent to give consent to the administration of puberty blockers," they wrote in their decision.
"It is doubtful that a child aged 14 or 15 could understand and weigh the long-term risks and consequences of the administration of puberty blockers."
Even for children aged over 16, they argued it may be necessary to involve the courts.
"In respect of young persons aged 16 and over, the legal position is that there is a presumption that they have the ability to consent to medical treatment," they wrote.
"Given the long-term consequences of the clinical interventions at issue in this case, and given that the treatment is as yet innovative and experimental, we recognise that clinicians may well regard these as cases where the authorisation of the court should be sought prior to commencing the clinical treatment."
In response to the ruling, the Tavistock Centre said it was "disappointed" and planned to appeal, but immediately stopped all referrals for under-16s.
The international LGBTQI community has been quick to condemn the ruling.
A petition calling on the UK government not to ban gender transition treatment for under-18s, which was started earlier this year prior to the court ruling, has reached more than 50,000 signatures.
"The government should not restrict trans health care treatments to under-18s in the UK," the petition reads.
"Treat these trans minors as human beings and do not use their age to justify restricting medical treatment."
The UK government had previously said it had "no current plans to change access to care or services for people under the age of 18 displaying gender dysphoria and wishing to transition to a non-natal sex".
Following the ruling, an NHS spokesperson told the BBC they "welcome the clarity which the court's decision brings".
"The Tavistock have immediately suspended new referrals for puberty blockers and cross-sex hormones for the under-16s, which in future will only be permitted where a court specifically authorises it," they said.
The move has sparked concern among the LGBTQI community across the world, including in Australia, worried that other countries will follow in Britain's footsteps.
Teddy Cook from ACON Health, which specialises in trans and gender diverse health equity, was critical of the move.
He emphasised that not as many people want to reverse their surgery as the media and conservatives would have people believe.
"More people regret lap band surgery than they do medical affirmation [treatment]," he told The Sydney Morning Herald.
Jo* is 20 years old and has fully transitioned into a man, after he was able to undergo hormone therapy as a teenager.
The Sydney resident had to go to court to get his hormone treatment approved, and feels sorry for his UK peers who will also have to jump through hoops for the therapy.
"It's a very visible step away from trans rights and medical care, and that's worrying," he told news.com.au.
However, conservatives in Australia have welcomed the move.
"The Keira Bell case further highlights the dangers of Annastacia Palaszczuk's 'conversion therapy' legislation," former Australian Christian Lobby managing director Lyle Shelton said on Twitter.
"In Australia, our courts allow a girl to have her breasts cut off while the British High Court has said no to giving a child puberty blockers."
*Last name withheld over privacy concerns.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
• For transgender support: Outline: 0800 688 5463
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