well i dont think there is anything wrong with having a baby at 45, i would just be very wary of making the decision to postpone ttc and then to find out that you couldnt get preg, if you wouldnt really mind not having another baby then that fine, but if its really important to you, then i would just get on with it, purely from the biological point of view that it is generally harder to concieve and unfortunately you are more likely to have problems if you leave it till your older :(
but we cant always plan these things, we dotn know how life will pan out, i was lucky enough to meet my dp at uni and we had our first when i was 20 (preg at 19) i am now 32 and we have been lucky enough to have 5 gorgeous healthy children, that is us done now tho! but i feel very lucky to have my family and as such i have just signed up to donate eggs so that i can hopefully give someone else the chance of having the joy that i have from my children, they asked me when i registered if i would mind my eggs going to an 'older' lady, ie in her early forties and i said no, not at all! who am i to be judge and jury on something like that, i would have no idea why she is in the situation of needing an egg donor, i am just grateful that i got lucky and had my own very easily and i simply feel that as i got lucky and i dont need my eggs anymore, why not give a few of them to someone who does?
sorry gone off on a ramble there, but ultimately do what makes you feel happy op, if you are happy to wait but run the risk of not having another (but wont ultimately mind) then fine, if however you REALLY want another i would get on with it, just because of biology and the fact that you are running the risk of more problems if you leave it :)