The 1850 census for Belmont County, Ohio shows William Harlen, son of Daniel and Margaret Harlen. The spelling difference is not surprising back then. This William clearly shows 7 years of age, which would mean born in about 1843. Although the "7" in his age slot is a bit faded, and could potentially be mistaken for a "1", it absolutely is a 7, as the family is listed according to age. His sister, Penina, age 9, is shown just before him and his brother, James, age 6, is listed immediately after.
Family trees do seem to show this same William as born in 1848, but my best guess is that they are mistakenly basing that on this census, which AT FIRST GLANCE, appears to show his age as 1, but it is without a doubt, a 7.
This would certainly be a CW veteran.
There was another William HARLAN in the 1850 Belmont County census, born 1849, son of Samuel and Eleanor, another reason why some family trees could be attributing a younger age to the other William, simply confusing the two.
Unfortunately, my access to fold3 through my local library is down at the moment, but I will check later.