Well when I have not practiced my violin for days, then my hands ache and cramp. And my shoulder/neck join feels weak too. So I gradually build up the amount of time spent practicing. For example the first day 10 minutes, then I take two minute's break, then practice another 10 minutes. Actually I just take a short break whenever anything aches. But if you continue practicing this way for a few days, you'll soon get into the hang of things and won't ache that badly even if you do one hour at a stretch!
I also do this exercise: slowly clenching very tightly and un-clenching, then stretching all my fingers a few times a day. It helps.
And always be aware of your hand condition when you practice. If you do something on the violin that causes pain or ache, that means you're doing it wrongly and should try different hand postions or ways and use the most comfortable one. I was taught this during my piano lessons, and have only learnt to really relax in the past few years and because of the relaxation, you'll be surprised at how much more you can achieve during a performance!
If you have the chance, you could go for a professional hand/neck massage. It does wonders for your aching hands and muscles! There is this really talented chinese sinseh in my town. One massage from him, and I no longer get the tingles in my hands (which I acquired after learning the violin and when I didn't know how to relax my grip on the instrument!)! Which improved my playing too, eventually.
And fret not, Gae, I'm about 8 years younger than you, and yet I've definitely felt the aches and stuff!

I think the kids who don't know how to relax feel it as much as we do!