The age of your inbound links
We have written recently about the importance of inbound links but just getting the links is not enough. Google looks at certain factors to determine the quality of your inbound links. If the links are all from paid link exchanges or spammy sites then they will not be valued and are rendered worthless.
But even if your links are determined to be legitimate, high-quality links there are further factors to look at. The age of the links is important and the older they are the better. Also, if the search engines see a ton of brand new inbound links (no matter the quality) they may be viewed suspiciously. Having many new inbound links can be the sign of a link exchange, which the search engines frown upon.
The best way to get those older links that are spread out in frequency, and are above, suspicion is to get started right now link-building.