You'll require a countersignature if you look markedly different since your last passport photo. In particular, the guidelines state that it's required if 'you cannot be recognised from your existing passport'.

Do children's first passports need countersigning?

When do I need to get child passport photos signed? If you're applying for a first passport for a baby or child, or a passport renewal for a child under 11, then you'll need to have the photos countersigned. This helps to prove the identity of the person applying for the passport.

What does countersign a passport mean?

The definition of countersigning a document is to provide your signature to a paper that has previously been signed by someone who isn't you. The countersignature can only be provided by a certain person, in the instance of a UK passport application, and is needed to prove that the application is an authentic one.