She was but 15 years of age.
Is this grammatically correct?
For more context:
Lavitsia sighed. As diligent as she was, she was but 15 years of age and attending to his maladies had become more than tiring for her.
The sense in which I try to use the expression "(is) but" is similar to that of "only" or "just" or "merely". As in, she was only 15 years old and all this pressure was soon becoming too much for her.
Note: For even more context, I'm trying to write a short story based on an earlier time period, hence the slightly stilted English.