"Young people" or other age limits are a dramatically distinctive case because the considerations involved are universal, cross-cutting and temporary. Age-based standards can't be used to create systematic outgroups. They bind and protect everyone equally in the ultimate and most practical sense because (a) everyone grows into enfranchisement (b) no one has it until they do. Which gives everyone an equal stake in what the specific age limit is too.

Contrast that with sex, religion, ethnicity, origin, asset ownership etc and the real question is why anyone would accept that they're truly comparable.

Citizenship may be the only truly stickier corner case. Largely because international relations will sometimes produce reasons to limit it more tightly or extend it more freely by broad categories such as national origin. But even here (a) most countries recognize that is often only one of several important judgements in a wise process (b) it's possible to review the equitability of citizenship qualifications: can they be met by anyone by way of objective assessment of investment in society and respect for acceptable participation?