There is no ideal or correct time for investment. Every time is appropriate if you want to invest long-term. Whoever bought at 2017 ATH can sell now and his investment will have been lost, but we can say that this time is the best time to achieve the maximum possible profit in the shortest possible time?
In theory, it is correct, but the price may be in the area between 25k to 32k at some point in the beginning of next year before the upward line continues. Therefore, now is an appropriate time to achieve maximum profit, but it is not the best.
To me i think it is never too late to invest in bitcoin, you can still make it in bitcoin as well, if you still want to invest in bitcoin now i will suggest that if you just buy it now you should keep it for the period of like 4 to 5 year luckily it might even rise beyond your expectation, so to me i think it is never too late to invest in bitcoin.