In order to comply with various laws across the world about restricting access to certain internet sites and services to minors, Google needs to check that users of its services meet the minimum age requirements set down by individual countries or, in the absence of any specific age for a particular country, set as a default by Google itself.
For example, Ireland requires that Google doesn't provide services to, or (in particular) retain any data about, people under the age of 16. The same age restriction applies in countries such as Bulgaria, Greece and Germany but other countries have lower age limits. (e.g. 14 in Spain in Austria). In countries, like the UK, where there's no statutory age limitation on the use of Google's services, the company still has an obligation to protect minors, so they impose their own default age limit of 13 years.
Google has faced criticism for only checking up on a user's age when they've suspected that an existing user might be too young to use their services. So now they're ensuring that everyone who uses their services is required to provide their date of birth.