For as many as are led by the Spirit of God, they are the sons of God.
Note 29 at Ro 8:14: Being led by the Spirit of God is the ultimate standard whereby we may know someone is a son of God. This raises the question, "If you aren't led by the Spirit of God, does that mean you are not born again?" It is true that everyone who is not born again is not led by the Spirit of God and that everyone who is born again is led by the Spirit of God, but that requires some explanation.
First, there are varying degrees of being led by the Holy Spirit. No believers are following the leading of the Lord as much as they could be, and if an absolute standard was applied to this verse, no believers would qualify to be sons of God. With this in mind, believers have been led by the Spirit to some degree in making Jesus their Lord, if nothing else.
Second, being led by the Spirit does not cause people to be sons of God, but being sons of God causes them to be led by the Spirit. All believers do have the Spirit of God to lead them, but that doesn't mean all believers heed His leading (see note 3 at Joh 10:3). In context, Paul had just spoken about denying the flesh through the power and leading of the Holy Spirit. Here he was simply pointing out that all Christians have the leading of the Holy Spirit available to them to accomplish this.
Note 30 at Ro 8:14: Some people have tried to make a distinction between being a "child of God" and a "son of God." They say a "child of God" is any born-again believer, while a "son of God" refers only to a mature Christian. This looks good on the surface, but further study will reveal there is no difference.
In Ga 4, these same terms are used, and "son" or "sons" is applied to all believers. Ga 4:6 says, "And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father," and Ro 8:9 says, "Now if any man have not the Spirit of Christ, he is none of his." Since every believer must have the Spirit of Christ (Ro 8:9) and every son of God has the Spirit of God's Son in him, crying, "Abba, Father" (Ga 4:6), then we can clearly see that every born-again believer is a son of God. Therefore, these terms are used interchangeably, and no doctrine about different levels of maturity can be drawn.