I would assume it would be the one which came out of the mother first. Both children would exit separately, so there technically would be one born before the other.

Thats what happened in Scotland anyway so i assume it would be the same for other nations. For example, Alexander (b. 1430) was the oldest twin born to James I of Scotland and his Queen consort Joan Beaufort, so he originally was heir to the Scottish throne. However, Alexander died in infancy, leaving his younger twin brother, who would become James II, heir apparent to the Scottish throne.

hi! there's room for more examples, but meanwhile, get started on this section of the FAQ*

How did inheritance work with twins?

*see the link on the sidebar or the wiki tab