My mom and I went on the Safari to South Africa. It was not an adults only trip, but I thought I'd chime in anyway. Just as a reference the kids on our trip ranged from 11-20 (the 20 year old was the oldest of several sibling on the trip). I was the next youngest at 23 and there was a couple with grandchildren, so maybe in their 60s. My mom might have been the eldest at 65. So the rest of the adults were likely in their mid 30s to mid 50s. I think it would be rather similar on the adults only trips. Do to the cost, younger folks won't outnumber you.

IMHO, anyway.
As to the restrooms: this is always a concern for me and I'm glad I'm not alone. I found it no problem at all. We never went more than 2 to 2.5 hours without a restroom and there is one on the bus should you have an "emergency" before the next scheduled stops. They always point out the bathrooms. Even once when we were told we would not have time at one of the airports, someone needed it and we were accomodated and no one was the worse for it.
The only issue you may have is on the safaris. They tend to last about 3 to 3.5 hours. There are no restrooms on the safaris. As Craig will tell you, every tree/bush is a lavatory. You simply let the driver know you need a stop and they do. Toilet tissue and hand sanitizer are provided by the guides. It's not glamorous and/or desirable, but our tracker and guides did use the tree lavatories. I found the best way to avoid needing them (I do not have Crohn's so I don't know how much this will help you) was to avoid coffee/tea in the morning/afternoon. There is coffee/tea in the morning before the safari, so I'd skip the drink and just have a cracker or two. After the morning safari, there is a true breakfast. Feel free to indulge here, you have several hours before the next safari. Then there's lunch, followed by afternoon tea. At this point, I skipped the tea and just had cake or cookies as a snack. After the afternoon safari, you have dinner. They also provide coffee, tea and hot chocolate on the safari. I'd also advise skipping these if you don't want to use the tree lavatories. I just waited until breakfast to have my coffee and had my tea directly after lunch rather than waiting until the appointed "tea time."
Have an amazing time! It is truly the trip of a lifetime!