How old or what weight? What carrier?

C is starting to get a little heavy in the front in my Beco Gemini, at 7.5 months and a little over 16 pounds-- not at all unbearable, but I thinking about when we will start doing back carries too. I put her in it once just to test it and it wasn't so hard to do myself (though I did it next to the bed, by the mirror!) but she seems so low, like she can't see over my shoulders yet, plus I like to be able to see her face and see if she is OK still.