According to Internet sleuths, Claire is born in February. So it's almost 2 years, but technically less. Justin turns 18 in November, and Claire will 19 until February 2021.
I actually would be more bothered if she were younger. Maybe it's just bias, but it's also that he is the headship and I'd worry a 19 year-old guy might feel the need to feel in charge and lead his partner. (Joshua Bontrager was older when he started courting, but he's a good example of this kind of mentality.)
It's a smaller age gap than Ben and Jessa. The biggest age gap in an underage Duggar courtship goes to Joy and Austin. Austin later revealed that they started courting when she was 17. He was 21 when she was 17. They didn't go public until she was around 19 and then they got married about six months later.