You can think of 'virtual limit' as a practical limit. It may not be fully impossible, but it is SO close to impossible that we call it 'virtually impossible'. If this is the case you can call something a 'virtual limit', but its not a very common expression. In the example above the writer is saying no matter how far we advance there will always be some limit on our lifespan.

virtual means-almost,but not exactly or in every way.