It would be great if Del Potro manages to get where Djokovic and Murray are now, by this time next year.
I know it doesn't work this way, but:
Ranking when turned 17:
Del Potro: #209
Ranking when turned 18:
Del Potro: #106
Ranking when turned 19:
Del Potro: ? (23-Sep 2007) With 18 years and 1 month, he is #91.
Both Djokovic and Murray made the Top 20 with 19 years and 5 months.
Del Potro will be 19 years and 5 months old in February of 2008.
This probably means nothing.
I don't like to quote myself, but I think this old post may be interesting.