1a.  Census Index:  U.S. Selected Counties, 1790 (Broderbund CD-311):

1790

Ely, Daniel

PA

Berks Co.

Reading Borough

p. 39

Generally, one has to be married to show up as a head-of-household in the census.  Assuming the minimum age to marry is 18, Daniel would have to have been born before 1772.

1b.  Family Quest Archives:  Pennsylvania Federal Census 1790:  Berks, Chester, Delaware, Huntingdon, Mifflin (part), Luzerne, Dauphin, Northampton, Cumberland, Fayette, Westmoreland (part), Bucks, and Lancaster Counties.  Heritage Quest CD M637-8 (extracted by Diana Gale Matthiesen):

1790

PA

Berks Co.

Borough of Reading

p. 1

Daniel Ely

2-1-1-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white males	16 or over	in or bef. 1774	= Daniel = Son A
1 free white male	15 or under	1774-1790	= Son B
1 free white female	all ages	in or bef. 1790	= Wife

The question here is whether "Son A" is their son or some other male (e.g., hired help).  If he is their son, then this is the picture of an older couple with their last two, youngest children still in the household.  If "Son A" is not their son, then it is the picture of a newly wed couple with their first child.

2.  F. Edward Wright.  1994.  Berks County Church Records of the 18th Century.  Vol. 4.  Family Line Publ., Westminster, MD (Broderbund CD-166).

XXVI.  READING REFORMED CHURCH The first Reformed church was built in 1753-1755.  The earliest existing register begins in 1791... Location:  First United Church of Christ, 611 Washington Street, Reading. Records: Pennsylvania State Library. Records of the First Reformed Church of Reading, Pennsylvania. Copied by Rev. A.S. Leiby, Pen Argyl., Pa. 1940... REFORMED CONGREGATION IN READING...

Margrethe of John Mc. Weid and Elizabeth, b. December 15, 1793, bapt. June 12, 1796. Sponsors: Daniel Ely and wife.

Assuming one had to be at least 18 years of age to sponsor a baptism, Daniel would have to have been born before 1778.