You’re claiming you printed these out, colored them in with crayons, enjoyed the experience, and at no point questioned whether you were infantilizing yourself?
As someone trying to get back into art, I've been considering using coloring books for shading practice - so I'm not constantly distracted by wanting to correct my own "bad" proportions, as I would be with my own linework. Granted, I'm more interested in the Call of Cthulhu coloring book linked elsewhere in the comments section, and I'd use different materials, but if you've got crayons - why not! More talented artists might amuse themselves with corrupting the original linework first: https://www.youtube.com/watch?v=EiCx1BIRQWc
Infantilizing adults can cause serious harm when it undermines their independence, or their ability to take on the burdens and responsibilities of adulthood. However, I'm hard pressed to see any harm in an adult enjoying a coloring book, especially when used as an opportunity to bond with the children they're raising or teaching. The original poster comes across as someone proudly exclaiming "Those are baby toys, I don't play with baby toys!" - a phrase and attitude I've heard almost exclusively from actual children - just dressed up in fancier words. I think it is far more infantile than a coloring book. Some of us are secure enough in our adulthood to indulge in harmless childishness from time to time. You can't stop us. Phhbbbbbt!! (Care to join us?)
Of all the different art supplies I've used, I think I like pastels and colored pencils the most. Painting is fun but usually involves more setup than I give myself time for.
I think you have a hangup on being 'adultlike' versus 'kidlike'.
No, I see even fingerpainting as a good diversion if the circumstances are right.
Lighten up and don't get so stuck on "once I'm an adult, <list of things> are forbidden to me".
