Why should more voters make FPTP a better system? That would only happen if those who at present can't be bothered to vote, tend to favour one party. Should a party be in power only because many of their voters have been paid to turn up at the polling station?
But If the current non-voters don't tend to favour a particular party, adding them in would just add more noise and make close results even more random.
Anyway the only people who really care about low turnout are politicians, because they think it makes government look less legitimate.
Personally I would reduce the number of voters by putting the age back to 21. Young people are just as intelligent and interested as older ones, and no doubt more so than some of the very old ones. But experience is considered necessary for all important jobs and voting is one of the most important things we do. It can decide peace or war, and nothing could be much more important than that.
Personally I was very interested and took it seriously when I first voted. But even at the grand old age of 22, I paid far more attention to political speeches and manifestos, and less attention to party records in government and opposition, than I would do now.