Zhoie started using it outside on her own full time at 5 mos and she is 2.5. Rain and snow are not her most favorite times, but she still doesn't go inside.
However, I do dress her for the weather. If it's raining, I put on a rain coat, if it's cold she has a winter coat. If we are going to play in the snow for a little bit, she has a snowsuit and boots. That pretty much takes care of it.
Wishing you the best of luck...try dressing for the weather.