Berlin Opera

Yes, it would be very safe to walk there. The walk takes you along Unter den Linden, which is one of the most popular and beautiful streets in Berlin, so there will probably be lots and lots of people out walking even in the evening.

Do keep in mind that it's about a mile and a half, so it would probably take you about 30 to 35 minutes, depending on how fast you walk. Regardless, I would have no qualms about walking it myself, and I'm female and older.

Berlin has more than one opera company, so I assume you refer to the Berliner Staatsoper. You can also hop the M2 tram on the corner at Prenzlauer Allee to Alexanderplatz. You could walk from there or take the 100 bus right to the Staatsoper.