Um, it doesn't say only 20 and up were atoneable, it says they were expected to contribute an offering. Obviously you have no clue as to what 20 years old and up meant in the Hebrew culture. It isn't a proof text against the fact all are sinners inherently.Ok so we should be familiar with the yearly day of atonement, and Christs work on the cross which replaces it. We agree there, right (romans 5:11)? I hope so. Here are a few verses about the day of atonement, and what was to be done, and why.
16 And he shall make an atonement for the holy place, because of the uncleanness of the children of Israel, and because of their transgressions in all their sins: and so shall he do for the tabernacle of the congregation, that remaineth among them in the midst of their uncleanness.
30 For on that day shall the priest make an atonement for you, to cleanse you, that ye may be clean from all your sins before the Lord.
10 And Aaron shall make an atonement upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make atonement upon it throughout your generations: it is most holy unto the Lord.
Well, in the instructions about the atonement in Exodus 30, the Lord told Moses this:
11 And the Lord spake unto Moses, saying,
12 When thou takest the sum of the children of Israel after their number, then shall they give every man a ransom for his soul unto the Lord, when thou numberest them; that there be no plague among them, when thou numberest them.
13 This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel is twenty gerahs an half shekel shall be the offering of the Lord.
14 Every one that passeth among them that are numbered, from twenty years old and above, shall give an offering unto the Lord.
Now go away.