Harvesting lemons

My lemon tree, which over wintered in my conservatory, had one small lemon on it, which was just turning yellow on one part when we put it outside for the summer. The lemon is still on the tree, but is the size of a small grapefruit now and yellow all over. I`m beginning to thing it likes where it is and doesnt want to leave its mother   

It`ll be my first crop from the tree

Is it ok to pick it now, or should I wait for it to fall off the tree ?

The tree has about 5 other very small lemons on it aswel at the moment.

I was told long long ago that a lemon is the only fruit that will not ripen once picked.

Ready to pick when it's yellow..