When Simon was in his (thirties),he founded his own company.
当西蒙30多岁时,他成立了自己的公司.
in one's thirties,在某人30多岁时.可不可以用thirtieth在某人30岁时？没有这样的用法。如果表示第几岁的生日才用序数词。in one's thirties是个习惯用法，指某人30到39岁之间的这段时间。