Allergies can certainly develop as you get older, because just like the rest of our body changes as we get older. So does our immune system, which means that sometimes our immune system was tolerant to something, but it slowly becomes intolerant to something as we age. So you can definitely get allergies to things like cats or dogs or ragweed as you get older.
And as always, the way you want to treat these is if you can avoid it, that's the. If you can't, you may want to try some of the over-the-counter anti-histamines and if you can't do that, there's always the option of immunotherapy, which can be taken in the form of either allergy drops or allergy shots.
Hope this helps. Thanks so much.