If it's still around when she turns 13. With the content and adult type of content on this page, I'm tempted to not show her til she's 18...
I've played since I was 13 and look how I turned out though

On a more serious note, there are sort of 3 reasons for and against...
For:
1. Daughter / father bonding
2. Artistic expression / Broader friend spectrum (I now know people in Europe, Australia, etc)
3. I'll bet $20 when she goes to school, by the time she's 13 none of the stuff on BL would surprise her. My middle school was a serious wake up call, and I knew every dirty word in the book before 5th grade. :L You can't truely protect her, you can only try and guide her, let her fly and hope that she soars. Whether or not she 'soars' is entirely up to her and her personality.
Against:
1. Again, the Block Land community can be a little harsh, especially to our female members
2. internet addiction 101, prime example right here ♥
3. Sort of going back to #1, she would eventually run into people like Furdle, Dotdot_____, etc. Long story short, they really don't need to have an influence on anybody.
You could, however, play with her? If your key is still active 12 years from now, you could get her to sit down and just play on a LAN server, if you have two computers you could play together on the same key. I'll be 29 by that time, but I bet I'll still be around :P
See if she has any interest in legos early on, I remember finding blockland because I googled "virtual legos," found this drama, then moved on from there to find the actual game itself.