Google has updated its store ratings overview for merchants, detailing specific eligibility requirements that include having around 100 eligible reviews. Advertisers must maintain a star rating of at least 3.5 stars out of 5 for these ratings to appear on paid text and Shopping ads.
The requirements specify that reviews must be from the past 24 months to ensure ratings reflect current service quality. Ratings are calculated separately per country, with reviews counting only within their origin country.

Read the official help page here: Google store ratings: An overview for Merchants.
This update was spotted by Hana Kobzová.