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svaret från @Praxis använder källan på awoiaf som säger:

Mace Tyrell is no more than 10 years older than Cersei Lannister, who was born in 266 AC, indicating that Mace was born in 256 AC. Any later seems unlikely, since that would make Mace's mother Olenna relatively old during the birth of her first son.
Mace was thus born in 256 AC.

Titta på källorna för detta ser vi att Cersei föddes i 266 AC:

What Tywin Lannister made of this is not recorded, but in 266 AC, at Casterly Rock, Lady Joanna gave birth to a pair of twins, a girl and a boy, "healthy and beautiful, with hair like beaten gold."
The World of Ice and Fire, The Targaryen Kings: Aerys II

Cersei tror att Mace inte är äldre än 10 år mer än henne men detta är inte nödvändigtvis sant (opålitlig berättare ) .

Mace Tyrell was no more than ten years older than Cersei, yet she thought of him as her father's age, not her own. He was not quite so tall as Lord Tywin had been, but elsewise he was bigger, with a thick chest and a gut grown even thicker. His hair was chestnut-colored, but there were specks of white and grey in his beard. His face was often red. "Lord Tywin was a great man, an extraordinary man," he declared ponderously after he had kissed both her cheeks. "We shall never see his like again, I fear."
A Feast for Crows, Cersei II

Från Cerseis beskrivning och födelsedatumet skulle det få Mace att vara född omkring 8-10 år innan Cersei så 258 AC till 256 AC men vi vet inte exakt när. Så även om källan till det andra svaret är tillförlitligt är det slutsatsen inte, slutsatsen görs helt enkelt som ett gissning och inte genom att titta på materialet mer.