It is not a question of physical age it is a question of maturity. And even being immature does not preclude you having a relationship with your soulmate. It only means that things will be difficult at least. As far as relative age is concerned, it is logical, being of the same soul, that two people who are soulmates will be of similar age. If the answer were yes that would eliminate people who have been paralysed below the waist.
Sex has nothing to do with a soulmate relationship or in any relationship for that matter.
If the sex is great then I am happy for you. Those who deem sex as a necessary function of a happy relationship are working from a base and physical nature. These people are prone to problems in their intimacy with one another. All this means is that you might want to take a look at who you are and why your relationships always seem to fail. If there is a problem that you have it will not automatically go away once you find your soulmate.