At 26 years old, Underwood is a full 10 years younger than the age Viall and our last Bachelor, Arie Luyendyk Jr., were when they had the prestigious gig. But as Viall told ET's Katie Krause at KIIS FM's iHeart Radio Jingle Ball, presented by Capital One, in Los Angeles on Friday, just because Underwood is young, it doesn't mean he's not ready for marriage.
"I mean, I was older, Arie was older. They're mixing it up, and you get a younger person," he said of the show. "Colton's what? 25, 26? There's plenty of people who are 25, 26, and get married in this world. I don't think he's, like, crazy young. I think there's a lot of people who get married at a young age."
"I think it will give it an interesting spin. I think it will mix it up a bit," Viall continued. "But I'm looking forward to seeing it. I think it will be fun. It gives it an interesting dynamic."
Viall ended his engagement to his final Bachelor pick, Vanessa Grimaldi, in August 2017, five months after his proposal played out on TV. As he told ET, it's the part after the show that gets "tough," but he recommends that Underwood really tries to put his new relationship -- if he's in one -- first.
"If he's in a relationship, they may or may not be engaged, but they clearly have a lot to learn about each other, and it's hard to do that in the confines of secrecy, and knowing what's to come from a show, but not knowing what will be shown. And that can be stressful and challenging, especially for the women, who have never experienced it before," Viall explained. "My advice is to be ready for the unexpected, and try to prioritize that relationship as much as you can, and it's tough. I wish him well."
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Nick Viall Reacts to Rumors Ex Vanessa Grimaldi Is Dating Peter Kraus (Exclusive)
Viall's ex, Grimaldi, was recently rumored to be dating fellow Bachelor alum Peter Kraus, and Viall previously told ET that he just wants her to be happy. As for his own dating life, he confessed it can be a little challenging to date after being the Bachelor.
"It's not a trust thing, [but] it does make it challenging. I think... I've gotten older. I think as you get older, you become more selective. It can be a little tough sometimes with trying to have people be comfortable around you, but it's fine," he insisted. "I'm dating, I'm trying to date, I've met some great women. Bachelor or regular person, I think dating can be challenging regardless. So it has its challenges, but I don't blame anything on the show or anything like that. The problem is me."
That's not to say Viall isn't trying, however. "My process is just to try to open as many doors and windows as you can. I think you have to try to say yes more than you say no," he shared. "Dating can be hard and discouraging, so I just try to be open."
The Bachelor returns Monday, Jan. 7 at 8 p.m. ET/PT on ABC -- join ET's Bachelor Nation Facebook group here.