Model releases are a legal contract between YOU and a MODEL only, agencies are nothing to do with it, they don't make the law even though you'd think they did. Agencies make demands about releases based on their own agenda, which they can do if they so please but that does NOT make it law.
If the legal age for signing a contract in Singapore without a parent/guardian counter signature is 21 then that is what you MUST comply with irrelevant of what any agency tells you.
Edited to add: The laws that apply to the release are those in which the shoot was conducted, so in your example if the shot was done in the UK but the shot was used in Singapore via an agency in the US, UK contract law applies.