True, although not to the same degree as seniors by far. I'm struggling to find much data, but https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1950160/ claims +55% male infant mortality rate in Paris and +12% in the rest of the country (females were unaffected, which I guess ties in with male infants generally being more fragile than females) during the 2003 heat wave in France. They don't give baseline rates, though, so it's hard to tell what this means in real numbers.