The suffix -ish simply defines almost, approximately.
- Can we say 49 as 50-ish when it is referring to numbers?
- Are 47,48,49,51,52,53 also referred to 50-ish?
According to Collins COBUILD Advanced Learner's English Dictionary, you can use -ish in three different situations. The third usage matches your scenario:
-ish is added to words referring to times, dates, or ages to form words which indicate that the time or age mentioned is approximate.
I’ll call you guys tomorrow. Noon-ish...
The nurse was fiftyish.
Considering the meaning of '-ish' i.e. about/somewhat, when it's used with time and age, to answer your specific question, I guess you should ask "when do we use about/almost?"
I think generally we use about 50 years old when,
We're not sure whether a person is 50 years old or not but his looks make us think of this approximate number. Like what witnesses do while giving descriptions to a police officer.
When we know the number but we want to round it up. Like 50 years and 2 months or 49 and 8 month but it's easier to say 50 (Here I'm not thinking of the real concept of rounding up in maths because it's based on clear rules already).
So when you say "fiftyish" you mean "more or less than fifty".