My first thought is that the age difference is definitely signifiant. Not bad, not wrong, not unworkable, but significant. At 21, your partner just hasn't been an adult all that long, and she's still finding out so many things about herself and the world which you've had a lot more time to sort out than she. It is commendable that you recognize this enough to appreciate that she may like to have the explicit freedom--stated by you--to continue her explorations without fearing the loss of your love and support in the bargain.
You said, "Apologies, as I realise I am asking more about single partner swinging that about Polamory." But why? Is it because you feel less threatened by her being sexual with another if there isn't a love-bond there also? You must realize that if you are wanting to provide her with your explicit acceptance of her need to explore love, sex, relationships..., that limiting her exploration to swinging is contradictory to your purpose?
Think of it like this, for one thing: Sex without love in it isn't really all that satisfying. At least it isn't to me, or anyone I know who is truly honest about it. Swinging isn't centrally about loving, but about sex. Polyamory is centrally about loving.
I recommend that you look carefully at any fear you may have around the possibility that she may come to love another AND you, both at the same time -- that she may like to choose polyamory. We don't know that she will, but you seem to be saying that it is okay with you for her to "swing" but not okay for her to open to loving another simultaniously with yourself. You haven't said this explicitly, but one could easily interpret what you have said in this way. Do you want to affirm her freedom and comfort and support her in being herself and exploring adult life? That, I think, would be the optimally loving approach. But doing so could stir up fears in you which you will need to address, as your own. And since you are partners, you should be able to talk about all of that with her and see how she can be loving and supportive of you as you challenge yourself to love in this way.
Finally, I'd recommend reading books and magazine articles, and on the web, on the subject of polyamory. There is a body of wisdom in "the polyamory discourse" which will benefit anyone in understanding loving relationships, whether they are poly or not.
I suppose it is clear in what I say that I disagree with a certain common notion of what constitutes "sexual liberation". As I see it, when it is good, sex involves whole persons, not just the errogenous zones, but also the heart and spirit and soul. The world is littered with broken hearts attempting to f**k themselves happy, but, as it has been often said, "you can't get enough of what you don't really want".