Ages from youngest to oldest
8/14 - Yanqing (Chinese version)
10/12 - Clara
10/12 - Hook
15+ - Sushang (character story)
19/20 - Arlan
19/21 - Asta
19/22 - March 7th
20/21 - Silver Wolf
20/23 - Pela
21 - Bailu (Vidyadhara)
21/22 - Fu Xuan
21/26 - Qingque
21/27 - Blade
22/23 - Trailblazer
22/24 - Tingyun
23/24 - Selee
23/25 - Dan Heng
23/25 - Bronya
25/27 - Sampo
28/30 - Kafka
32/35 - Himeko
35+ - Jing Yuan
36/39 - Natasha
35/40 - Cocolia
35/40 - Serval
82 - Welt (official lore)
Unknown: Herta (puppet)
Welt was born in 1947 on Earth, and his birth name is Joachim Nokianvirtanen Yang. The story of Honkai: Star Rail takes place after the events of APHO and is covered in the Alien Space Manga (year 2029, so Welt is 82 years old).
Decelerated aging is a remnant from when Welt was in possession of the Herrscher of Reason’s core. Using the reconstructive powers of the Core of Reason, he was able to replicate his own body cell by cell, so he “stopped” aging because his new body isn’t the “real” one.
Serval is the same age as Cocolia. They studied together and were “roommates”.
Cocolia adopted Bronya when she was in her 20s, about 12-15 years ago.
Gepard is only 2-3 years younger than Serval, as shown in a photo of them when they were children.
Natasha seems to be in her late 30s based on what she says about how long she was sealed off in the underground, and she was already an adult doctor when that happened.
The Chinese 1.0.5 version of the game stated that Yanqing is eight years old. The developers removed the exact age in a later version and replaced it with “young”, which might indicate a slightly older age as well.
Sushang is 15 at the beginning of the game. Character story, part IV: “Firstly, when making friends, you should consider if they’re good or bad. Sushang, you’re now 15 years old, but you still don’t understand how unpredictable and dangerous the world can be.“
Dan Heng is shown drinking in official artworks.
Asta might be at least 700 years old if she built the Herta Space Station. However, we don’t have accurate info on this topic.
Bailu is a Vidyadhara, a long-life species, so she ages slower.
Herta appears in the form of a puppet she modeled after her younger self, so we can’t estimate her real age.