Halfway up the stairs

Halfway down the stairs Is a stair where I sit:
There isn't any other stair quite like it.
I'm not at the bottom, I'm not at the top:
So this is the stair where I always stop
A.A. Milne

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This is the stairgate arrangement we have in our hallway. Because we have always had odd shaped stairways, we haven't been able to use normal, fixable stairgates. This one is three sections of a Baby Dan playpen. We've never had to actually fix it to the wall as the Podlings never seemed to twig that they could just push it out of the way. Until now! Several times in the last few days, Lily has followed me upstairs when the gate was shut, and this morning I heard her inform me "I upstairs!" in exultant tones when I was in the playroom and thought she was in the kitchen. She simply moves the gate away from the wall and squeezes through the gap. So, the question is, what do we do now? Do we nail the thing to the wall and scupper any future escape attempts, or do we move on to 'The Next Stage" and get rid of the gate all together? Both children are able to go up the stairs with confidence, but I like to accompany Lily downstairs just to be on the safe side. I'm looking for some advice from the parents amongst you. What age/stage did you allow your children free reign to go up and down stairs on their own? Is Lily too young to be allowed to use the stairs if I'm not right there with her? Am I worrying too much?! I really would like some advice here, so please leave a comment and let me know what has worked for you and your families.

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PS. Pardon the paintwork in the hallway. One of the reasons we could afford this house was that it was in poor decorative order. We've been here a year now, but still have a lot to do. We figure there isn't much point doing too much while the Podlings are young and in full destruction mode!