On the surface of things it is not a big deal (unless it's a potential statutory) issue. If someone is 50 but looks 40 and initially claims to be 40 I don't think it's a "deal breaker" in itself. Truth is I never looked at dating as trying to find a spouse. In fact like a lot of guys I told myself I was (never) going get married. I was wrong! :-)
I think having that mindset liberated me from a lot of the pressures put on the dating experience. Chemistry and attraction as well as relationship goals are far more important than age in my opinion. Raquel Welch (actress) is in her 70s and is still considered "hot". I'm sure she'd have no problem finding guys 20 years her junior wanting to take her out.
I suspect people are more likely to lie about age and weight via Internet dating. The primary reason would be to (increase odds) of their profile coming up in (age) range selcted searches. Even if for examaple a 45 year old woman looks younger than her age she is likely to be (viewed more) if she lists herself as being age 39. It's possible her experience has been guys her age or older are less "fun" to date. Therefore she tells a "white lie". It's not a big deal.