Tl;dr: Mydei is likely around 26-27 years old and Phainon is between the ages of 27-29 years old. That puts the Chrysos Heirs ages as follows from oldest to youngest: Tribios (oldest), Aglaea, Cifera, Cerydra/Hysilens, Castorice, Anaxa, Hyacine, Phainon, and Mydeimos (youngest)
Disclaimer: This is all conjecture and my personal analysis, so take this with a grain of salt. Itās also important to remember that the birth years for the heirs vary from cycle to cycle, so for the sake of this analysis, Iām going to only be talking about the 336 cycle (the cycle that was written into AIW and turned into the data used for Amphoreusā rebirth. This means that after Amphoreus is realized, the 336 cycle will basically be the ācanonā cycle).
Honkai is terrible at giving us actual dates and ages for Amphoreus, which means we need to take a lot of the clues given to us in canon and figure them out for ourselves. Letās look at the canon dates that we do have first. We know that Mydei left the sea of souls at 9 and spent 10 years with the Kremnoan Detachment before killing his father at 19 (source: AIW). We know that Mydei spent time in Kremnos after killing Eurypon since he himself stated that he lived in Kremnosā inner city for a few years during the 3.1 storyline. Mydei was also familiar with the layout of the city, enough to not need a map, so letās say he was in Kremnos for a minimum of 1-2 years before moving his people to Okhema (this would also explain why all Kremnoans, not just the ones from the KD, are familiar with him and acknowledge him as their king. They wouldnāt do that if Mydei hadnāt spent any time with them after killing Eurypon and moving them to Okhema). In the 3.1 story we see Mydei playing with Kremnoan kids who were all born in Okhema but old enough to play on their own (with Kraterosā supervision), so they would have to be at least 4 or 5. After adding a year to allow for the conception and birth of the kids, this would put Mydeiās age at a maximum of 26 or 27 years old. Personally, I view Mydei as being 26 years old at most.
Phainon is a lot more tricky to pin down. Phainonās story in 3.4 tells us that he was between the ages of 13-15 when Aedes Elysiae was destroyed and he wandered until he was an adult and found by Aglaea, so he wouldāve been around 16-18 years old when he came to Okhema (boys became men at 16 in Ancient Greece, but we donāt know if Amphoreus is the same so Iām leaving room for error). This is where Phainonās age gets tricky to extrapolate. We know from readables that after he moved to Okhema, Phainon went to the Grove to study under Anaxa and participated in at least 10 Great Debates. Side note, but this is where Hyacine was confirmed to be older than Phainon. This is because she was already working as Anaxaās TA at the Grove when Phai started studying there. If Hyacine started working as a TA when she became an adult at 16/18, that means she would be at least a few years older than Phainon since, as stated earlier, she had already worked under Anaxa for a while before Phainon came to the Grove. That puts Hyacine currently somewhere in her late 20s to early 30s, she just doesnāt look like it because sheās very short (rip).
Getting back on track. Itās unlikely that the Debates happened once a year because that would have meant that Phainon stayed at the Grove for a decade (since he participated in at least 10) and would be in his 30s at the earliest. A much more likely conclusion is that the Grove functions like a university with two semesters per year and the Debates happen once a semester, meaning Phainon studied at the Grove for 5 years before graduating and returning to Okhema around the age of 21-23 at this time. This timeline is the most likely option since Phainon had to have been in Okhema when Mydei and the Kremnoan refugees arrived 4 or 5 years ago, otherwise their famous 10-day battle wouldnāt have been possible if Phainon was still at the Grove.
After returning to Okhema at ~21-23, we know that Phainon joined the royal guard and officially took up the mantle of Chrysos Heir full-time. I believe that he spent at least a year training before Mydei arrived in Okhema, since I donāt think they wouldāve sent a green soldier fresh out of the Grove to face the Prince of Castrum Kremnos in diplomatic negations lol. Therefore, Phai probably spent 1-2 years training before Mydei arrived at Okhema. Since we established earlier that itās been around 5 or 6 years since their 10-day battle (going off the ages of the Kremnoan children born in Okhema), that puts Phainon in the age range of 27-29 years old. Personally, I view Phainon as 27 years old and 1 year older than Mydei.
That puts the Chrysos Heirs ages as follows from oldest to youngest: Tribios (oldest), Aglaea, Cifera, Cerydra/Hysilens, Castorice, Anaxa, Hyacine, Phainon, and Mydeimos (youngest)
Again, this is all my personal conjecture and not meant to be taken as canon. If anyone has any other Iād love to hear them! I love yapping about my favs lol