Sometimes you find 〇 being used together with kanji numerals (一, 二, 三, ...) to signify "zero".
In horizontal writing it is more common to express numbers using Arabic numerals (0, 1, 2, 3, ...) — and Arabic numerals are often also used in vertical writing.
一〇年以上 = 十年以上 = 10年以上
jū nen ijō
more than ten years
The official answer is: the 旧字体 kanji are those that were officially matched to the corresponding simplified forms.
The official List of Jōyō Kanji by Japanese Agency for Cultural Affairs has them in parentheses after the main entries.
The official List of Jinmeiyō Kanji by Japanese Ministry of Justice contains two kinds of 旧字体: in the list itself (these ...