“Some people may see this as a victory for children, but we’ll actually see a restriction in the services that start-ups can build for kids,” said Dom Hallas, executive director of Coalition for a Digital Economy, an advocacy group for start-ups that has received funding from Google, Intuit and Stripe.

Ms. Denham did not weaken the rules in response to the industry pressure, but she did clarify and amend some provisions. For instance, the final code suggests that instead of trying to determine a user’s age, online services could just apply the standards for children to all users.

The new rules, called the Age-Appropriate Design Code, are intended to give minors in Britain special rights and protections online — much like in the real world where children generally have the right to attend school and are prohibited from going to bars.

“We already treat children differently in the offline or analog world than we do adults,” said Ms. Denham. “So why shouldn’t we also treat them differently in the virtual world?”

The code lays out 15 different principles that sites, apps and other online services likely to have users under 18 in Britain must follow. Among other things, it prohibits such services from influencing minors to share unnecessary personal information or select weaker privacy options.

It also requires sites and apps to collect as little personal information from minors as possible. And it prohibits online services from using children’s personal data in ways that could be detrimental to them, such as by automatically recommending sexual or violent content based on their searches. That is of particular concern in Britain, where a teenager, Molly Russell, committed suicide, which her family said was influenced by her seeing images of self-harm on Instagram. Instagram subsequently banned images of graphic self-harm.

The new British children’s rules may require online services to make cultural changes as well as practical ones. The code requires online services to put the best interests of children first, above their bottom lines.