It's impossible to predict a future spouse's age from a birthchart with any guarantee of accuracy. The best any astrologer can do is an educated guess.
The seventh house, its ruler, and any planets in it describe your spouse (if you have one) in very general terms: their character traits, sometimes some detail about them. More accurately, those placements describe who you tend to draw into your life as a partner, who you tend to be attracted to as a partner. Your spouse will fit them in some way, but there are many possible ways.
Saturn is the planet associated with age, and with people much older than you. If it's your seventh house ruler and/or placed in your seventh house, a significantly older spouse is one possible manifestation. But not the only one, not by a long shot. Saturn could also mean something like, your spouse is deeply committed (or that your own approach to relationships is to be slow to get involved and deeply committed when you do) or has a burdened life, or carries a lot of responsibility.
Even when Saturn/seventh house does manifest as an older spouse, it doesn't specify how much older. Any age difference that takes them out of your peer age range qualifies. That may mean 20 years if you don't marry them until you're well into middle age, but not necessarily. The younger you are, the less of an age difference it takes to be a significant difference, and there's nothing in a birth chart that specifies when you'll marry. I don't understand how your friend came up with two decades. There's nothing in the birth chart that spells that out.
Saturn can be considered your seventh house ruler because it's the traditional ruler of Aquarius. Its placement at your midheaven might indicate a spouse who's in a position of power, relative to yours: accomplished in career, the boss at work, something like that. Achieving that kind of position tends to come with age, which could be another suggestion of an older spouse. But again, that's not all it could mean. Perhaps your spouse will simply be someone you meet through your career. Or someone who is accomplished and in charge and is your peer.
But I'd give more weight to Uranus, because it's in your seventh house, and it's also a ruler of Aquarius. Uranus, as an indicator of partners, doesn't have any particular association with age. Instead, it indicates an unusual approach to relationships, an unusual kind of marriage. Possibly multiple marriages. Possibly a need to share a cause with your spouse. It also indicates a need for space in your relationships.
At the end of the day, who you marry is your choice. If you don't want a spouse 20 years older, don't marry anyone 20 years older. There are plenty of people closer to your age who fit the parameters of your birth chart.
