Telling an older woman that she "used" to look really pretty can be perceived as both a compliment and an insult, depending on the context and how it is delivered. The way you phrase such a statement and the intention behind it are crucial in determining how it will be received.
Compliment: If said with genuine admiration and sensitivity, it could be taken as a compliment. For example, if you express it as, "You have always been so beautiful, and your beauty has endured over the years," it can be seen as a kind acknowledgment of her attractiveness throughout her life.
Insult: However, if the statement is said insensitively or with an implication that her beauty has faded with age, it can be hurtful and insulting. Saying something like, "You used to be so pretty when you were younger," may be perceived as insensitive and draw attention to the idea that her looks have diminished with age.
Aging is a natural process, and while someone's appearance may change over time, it is essential to be respectful and considerate when discussing these matters, especially with older women. Many people prefer compliments that focus on qualities beyond physical appearance, such as intelligence, kindness, wisdom, or accomplishments.
In general, when complimenting someone, it's a good idea to focus on positive attributes that celebrate who they are at present rather than drawing comparisons to their past appearance. Being sincere, respectful, and uplifting in your comments is the key to ensuring your words are taken as compliments and not as inadvertent insults.