Translation:I estimate her to be around fifty.
Does anslår always require til? If not, what would it mean/how would you use it without the til?
When an estimation is added, and the sentence is structured as the above, 'til' is required.
At least in proper writing, in spoken language omitting it would be fine.
Another option is to rewrite the sentence in this fashion, using 'at'; "Jeg anslår at hun er...", but when applying that sentence structure it's also common to use verbs like "tror" or "antar".
When the sentence only deals with the act of estimating, and there is no actual estimation present in the sentence, neither 'til' nor 'at' should be used.
Just when you require 'to' in English.
"Jeg anslår at vi trenger fem" = "I estimate that we need five"