With it being outdoors and at this time of day, you have some choices. You could go casual, dressy casual, or semi-formal.
It also will depend on your venue. Do they have a dress code? Is it at someone's home where they would have a preference of one or another?
What are you and the groom wearing? If one or both of you are wearing jeans, it would probably be casual. If he's wearing slacks and a button down shirt and you're wearing a dress or dressy pants outfit, you would want to go with dressy casual. If he's wearing a tux and you're wearing a traditional wedding gown, it's semi-formal.
Also, consider what kind of food you are serving. If it's a BBQ, you don't want to go semi-formal, but you could go casual or dressy casual. If it's appetizers and cake, you would want to go dressy casual or semi-formal.
If you haven't bought and/or sent out your invitations, this is usually the guests first clue as to the formality of the wedding. If it's a cute little cowboy and cowgirl, they'll know it's casual. If it's the traditional invitations with velum between the inside of the card, white and gold and/or black, etc., then it's formal. And anywhere inbetween.
It's always a good idea to have a website. That is where you'll put information such as the formality of the wedding, maps to where it's at, schedules for events surrounding the wedding, the places you've registered at, etc. With your wedding being anything but formal, you can put a business sized card inside the invitation with the website address on it.
Summer suits are not inappropriate due to age. They are inappropriate due to the formality of the event. Men can wear jeans and a nice shirt for casual, slacks and a button down shirt with or without a summer jacket for dressy casual, etc. Women can wear jeans or slacks for casual, slacks or a skirt and a nice blouse or a more casual dress for dressy casual, a skirt and nice blouse, a pants outfit, or a dressy casual dress for dressy casual, etc.
One thing to warn your female guests of on your website is that the wedding is taking place outdoors, so they may be more comfortable not wearing stilleto heals since they tend to stick in the ground. Depending on your venue, the people there may frown on that, as well.
I wish you all the best!