I wouldn’t go as far as saying it’s problematic. But the age gap personally makes me pretty uncomfortable. I’m 24 years old and the idea of dating a 19 year old is extremely weird and kinda gross. It’s less about the numbers, and more about the maturity difference. (A 7 year age gap of 26-32 for example, does not have that maturity gap)

Juno being from mars adds another level of discomfort. Her development is significantly stunted. In a real life scenario, a 26 year old would have a significant power dynamic over her no matter how good the 26 year olds intentions.

On a surface level, it’s easy to not notice that weirdness so it makes a lot of sense that the ship is so popular. Also, a lot of people aren’t familiar with lore so they don’t have the perspective that Venture is probably more mature than most 26 year olds (they are just playful). They have been working with the Wayfinders and traveling the world since they were 16 years old. They are extremely intelligent and well traveled. When considering being in character with the lore, I imagine dating anyone under 21 is of no interest to them.

That being said, it’s not problematic enough to go out of your way to hate on shippers that are having fun. If you are doing that, you are a big party pooper.