Here’s an expert example of using ‘their’ with a general reference:
The use of ‘their’ is confusing to give level to, because there is differing information between the English Vocabulary and Grammar Profiles, and secondly the uses are hard to locate in corpora automatically.
At B2, Point 58 in the category of DETERMINERS & possessive is defined as:
GENERIC ‘THEIR’ before nouns to refer to a generic body or group of people expressed as a singular subject
Everyone has their own opinion. …everyone has their preference…
In the EGP, Point 50, B1 examples:
They were having the time of their lives. They filmed the artists and their work.
However, in the EVP ‘their’ is:
So formally we can investigate for indefinite pronouns to the left of ‘their’. But first, we can check which _pn1 might work.
1 ONE 10647525
2 SOMETHING 7038440
3 EVERYTHING 3661675
4 SOMEONE 3447852
5 ANYTHING 3347793
6 EVERYONE 3007402
When we do a collocate search up to 4 to the right, every example was together on iWeb.
Does that qualify as a singular subject?
7 NOTHING 2966176
8 ANYONE 2741077
9 NO 881566
10 EVERYBODY 476935
11 NOBODY 451477
12 SOMEBODY 350684
13 ANYBODY 307399
14 NO-ONE 54132
15 SOME 32764