When to change road bike I own this Fuij Roubaix 2008 model road bike, its an aluminium frame with carbon front and rear forks. I already did 25000km on the bike and replaced wheels, chainring, a couple of cassettes, chains and so on.

Will the frame wear out? I am only concerned that the frame will ware out and fail during riding, otherwise I like the bike since its suits me, the size is just right and I like the non-sloping frame.

I am concerned about frame because the rear bolts between the carbon seat stays and the aluminium chain stays were lose and there is this mechanic which every time it sees me with the bike says that its going to break.

What do you think? I would avoid buying another bike not because of the cost but because I like my bike and to the new one I would have to adjust a bit.

Basically I am asking how to know when to replace a bike or a bike's frame,
25000 km isn't a lot. 

There isn't a time or mileage you should consider swapping out a frame. Instead, you should look for damage, such as dented tubes or cracks. You should be especially wary if you've been in an accident to check your bike out. Carbon fiber complicates this a bit, because its failure modes are generally more rapid than metal and harder to detect damage visually.

The mechanic might be trying to sell you something (most likely) or he might be right. You may want to get a second opinion from another mechanic.