All you can do now is wait til after the last frost in spring and put it out then, for life. Either it'll make it or it won't and there's not much you can do to change it at this point as you have not said where you live, so I can't comment on whether you can even possibly acclimatize it to being outdoors right now or not.

Without knowing more about your apple tree (and where you live), a winter hardy tree can usually skip a winter now and then without horrible consequences (it is almost as if the winter had been unusally mild). However the cold is often necessary for its reproductive cycle, so without the winter you might not get flowers, fruit, etc. Likewise, kept indoors for too many years and the tree will weaken each year and most likely die (depending upon the species). Here in Southern California some stone-fruited trees like plums, apricots, etc, will only flower and bear fruit if they are planted on a North-facing slope in a cool low area (like bottom of canyon) where winter temps get down into the 40's. However they will LIVE regardless of where you plant them (just not flower and fruit). For a little seedling I probably wouldn't worry too much about it.

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