"Eternal Life" in Matt. 19.29 points to the kingdom. For eternal life is seen in 3 stages:

(1) in this age - "hath eternal life" (see John 5.24)

Jhn 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.

(2) in the millennial kingdom - that is, in the age to come (see Mark 10.30 and Luke 18.30)

Mar 10:30 But he shall receive an hundredfold now in this time, houses, and brethren, and sisters, and mothers, and children, and lands, with persecutions; and in the world to come eternal life.
Luk 18:30 Who shall not receive manifold more in this present time, and in the world to come life everlasting.


(3) in eternity (see Rom. 2.5-7)

Rom 2:5 But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;
Rom 2:6 Who will render to every man according to his deeds:
Rom 2:7 To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life:
[He will give eternal life to those who persist in doing what is good, seeking after the glory and honor and immortality that God offers.-NLT]