It is true that under the old covenant a Levite would only serve from 30 to 50 years old. But you have to remember what the Levites did, they transported the tabernacle and all the various furnishings that went with it (Numbers 4; Numbers 8). Then when David made preparations for the temple, the Levites were assigned different duties: gatekeepers, singers, etc. (1 Chronicles 23).
But how does that apply to the new covenant Christian? We know from history that the apostle John was active in ministry into his 90s, although much of that was spent in exile. The apostle Paul would probably have been in his 60s because he was a contemporary of Jesus and the apostles. Consider too the example Anna (age 84; Luke 2:36-37) who spent her days in prayer in the temple.
Yes as we grow older we can’t do many of the things we could do before but that doesn’t mean that we stop serving the Lord. Paul urges us to persevere until the end.
Philippians 3:11-14 NASB 11 in order that I may attain to the resurrection from the dead. 12 Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. 13 Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead, 14 I press on toward the goal for the prize of the upward call of God in Christ Jesus.
Paul also teaches that no matter what we do we should do it as unto the Lord.
Colossians 3:17 NASB Whatever you do in word or deed, do all in the name of the Lord Jesus, giving thanks through Him to God the Father.
Colossians 3:23-24 NASB 23 Whatever you do, do your work heartily, as for the Lord rather than for men, 24 knowing that from the Lord you will receive the reward of the inheritance. It is the Lord Christ whom you serve.
So we should never think that just because we reach a certain age, we should stop serving the Lord. God’s retirement plan is heaven. It is then that we will experience the final peace promised by Jesus.
On a personal note: while lately I’ve struggled to find a full-time occupation to help support my vocation of being a child of God and disciple of Jesus with the gifts of teaching and preaching, one thing I do know is this, I have no aspiration or desire to go into politics. I’ve become too opinionated and have come to the place in my life where I am unwilling to compromise in too many areas and wouldn’t do well in that arena. I don’t believe that God has that in store for me either.