ANSWER - To my knowledge and I'm pretty certain of it, there is no spell that will actually reverse the ageing process. You'd be hard pressed if I'm honest to even slow down the ageing process and even if you did employ magic you would only be masking the symptoms, not actually slowing down the years themselves.
Which makes it important for you to understand what it is that you are actually asking for. A spell to slow ageing could be for the purpose of any of the following:
1. To reduce the age gap between you to one that is more socially acceptable.
2. To change his looks so he looks younger and more attractive.
3. To enable you to spend more of your lives together and make it so you are likely to die at a similar age.
If we look at the first... To be honest a love that can't withhold a little eyebrow raising probably isn't a love worth having. If you care more for what people think about you than you do for the relationship then chances are the pair of you are going nowhere. The exception of course would be if you were underage and not legally old enough for an adult relationship. In your case as you have indicated you are not in your teens then I see no reason why society should balk at you having a relationship with a man in his forties. A strong love should be able to overcome the petty judgements of those who believe otherwise.
My rationale for the second point is a similar one. Love should supercede a few grey hairs and a pot belly. If you don't think you'll love him when he's old and incontinent then chances are you won't last the distance. Older men may seem glamorous now but project into the future when you're in your prime at 40 and he's looking forward to his bus pass. You've got to be certain that what you feel for him goes deeper than his looks and if you're struggling now, it will get harder as he heads into the drooling years.
The third point is the only one for which I would do an anti ageing spell if it was possible. It is very hard indeed to love somebody and know your time together is limited by the age difference between you. However the poets remind us it is better to have loved and lost than never to have loved at all. So - if he's worth it - embrace the time you do have together and accept that it may be short but it may also be wonderful. If your love can't overcome the fact that you will lose him one day and be left on your own then perhaps you would do better to cut your losses now and save yourself the future heartbreak.
Incidentally I do empathise, my own husband is 18 years than myself and we have faced quite a few similar dilemmas. My own choice was to go with it and accept that the sexy man I met in his forties would one day become old long before I did. If you make the same choice then I wish you well and I wish I did have a magic spell to offer you. But as I can't offer the spell you want I'll just suggest you make sure you think it all through very carefully before you make a permanent commitment to a man so much older than yourself.
Image http://www.freeimages.com/photo/1196167 (wedward)