He's too young to be a Heroic Spirit. The only thing that makes Emiya special is his reality marble.
No one is too young or too old to be a Heroic Spirit. The Nasuverse is made up of infinite timelines where even heroes from the future can be summoned. Geronimo, especially with his name resounding throughout the world with that weird phrase, is definitely eligible as a Heroic Spirit.
Indeed, no one is "too young" to be a Heroic Spirit these days; Geronimo comes from an age where there was still some mystery and wonder left in the world, where legends about men doing crazy shit couldn't or wouldn't be dismissed as a tall tale. About the only requirement for ending up as a Heroic Spirit is that you have to be, in some fashion, legendary, and the other nuances and requirements have gotten a little more obfuscated now that we've got the likes of Billy The Kid and Thomas Edison also running around doing their thing in Grand/Order's canon.
Indeed, no one is "too young" to be a Heroic Spirit these days; Geronimo comes from an age where there was still some mystery and wonder left in the world, where legends about men doing crazy shit couldn't or wouldn't be dismissed as a tall tale. About the only requirement for ending up as a Heroic Spirit is that you have to be, in some fashion, legendary, and the other nuances and requirements have gotten a little more obfuscated now that we've got the likes of Billy The Kid and Thomas Edison also running around doing their thing in Grand/Order's canon.
No, the other requirement is that you be old enough to be in public domain if fictional.
The thing that destroyed human potential and capability for legend is apparently Steamboat Willy.
There is if you want to avoid getting sued by copyright holders and the estates of the deceased.
Hence the "public domain" thing. In order to be eligible to be a Heroic Spirit, you must be old enough to not require Type-Moon to pay somebody royalties.