My wife really enjoys anal and has some huge orgasms as a result. In fact she can orgasm without any clitoral stimulation during anal, just my cock doing its thing.
We've noticed its happend more as we've got older ..... which is nice 
Is this a common thing?
I think you learn more about what your body likes and responds well to with practise and age, so I'd say huge/better orgasms are probably very common. I'm in my 20's and have the same experience as your wife with anal. For us it is fun to include a clitoral vibrator anyway for maximum intensity orgasms!
We are early 60's so know each other well enough lol.
My wife sometimes uses a vibrator on her clitoris but not often. I find it facinating that a woman can orgasm with no clitoral or vaginal stimulation ...... its a wonderful thing 
Not sure if I'd class it as common as sadly most women cannot orgasm through penetrative sex alone (in the vagina) let alone through anal sex.
For me however I can and through anal sex too. I feel it (penis/toy) rubbing against my Gspot (which may be what your wife can feel). I can orgasm through anal sex alone however I climax harder with a vibrater on my clitoris as well.
Its good to read people in this age spectrum are enjoying themselves in this particular subject. It is a wonderful thing yes and ladies parts are quite spectacular at what they can do 
🐼
My partner comes hard from anal only!
My wife can also orgasm through anal sex only but using a vibrator or bullet vibe at the same time heightens her orgasm far more
Had anal sex last night and my wife had multiple orgasms through anal only
i thought anal orgasms for women were a myth til I had one. Didn't need clitoral stimulation at all whichleft me very surprised, along with jelly legs after!
I think it takes practice, I know for my partner it took a while before she started coming from anal penetration alone. I think it takes some time for your brain to recognize the stimulation and respond accordingly, you also have to be open to it so to speak lol. Practice practice practice!
I get intense orgasms from anal alone and leaves me with jelly legs.