Actually, I am self debating about this question when I started this blog. I tend to prefer putting advertisements in the later stage, maybe after getting a pagerank or acheiving a certain amount of alexa ranking before having advertisements. There may be a lot of blogs out there which is doing quite well even though they are practically a newly formed blog.
I think that reasonable amount of advertisements on a new blog is acceptable, but not too much until the reader is scare away. Personally, I have quit following some blogs because they are filled with advertisements, and sometimes concluding that the blog post could be a post for advertisements too. Credibility of the author will reduce when things like this happen.
If the ads are well-placed, not bring excessive and the ads are not those popping here and there ads, it will be fine for me when bloghopping.
For paid posts on my new blog, I will only accept sites that allows me to write freely about their site without having them setting the tone for me.
True, advertisements have to be in reasonable amount. Especially for new sites.