I heard that people, until the age of 25, will exhibit traits from their sun signs but after the age of 25, will take on traits from their ascending signs. Is this true?

I never heard of the specific age of "25" before, but I have heard about the theory that you "grow" into your Ascendant sign. Meaning, when you're a teenager, you exhibit your Sun sign, then as you grow into an adult, you become more like your Ascendant. However, I don't know the exact ages... but 25 doesn't sound too far off. I'd say people start acting like their Ascendant in their early twenties still (I've experienced it that way). But I think it's more or less based on the person's individual rate of maturity.

p.s. People always exhibit traits of their Ascendant (it is engrained in us from birth!) However, the theory is that people start really identifying with their Ascendant more and having the traits, sort of, override the Sun sign traits (becoming probably the most prominent energy).

You need to understand there is far far more to astrology than just ‘sun signs’ firstly there are 10planets in the sky and 12houses and any one of these 10planets can be in any one of these 12 houses – so there’s 144 possible combinations there –

What is vitally important is your ‘time of birth’ and nearest town and then go to free website like astro.com to calculate your natal chart, you may have to reduce the image in Extended chart selection to 63% save to your hard drive, then upload here.

Basic astrology
Now basically your sun sign is your 'core beliefs, values, ego' and Asc is your physical body, mode of expression and mask and persona that we all wear and hide behind, but definitely not what's going on underneath. What's going on underneath is your Sun sign. Moon sign, house placements and aspects describe your emotional and intuitive responses, also describes your view of your mother, as saturn describes your father by sign and house position.

So, think of a glass milk bottle the bottle is your Asc and your sun the milk, but the milk still has to 'come out' through the glass neck of the milk bottle....

Now aspects within a natal chart represent 'facets' of our personality, because we are all multifaceted beings. What these do is 'break down and compartmentalise' these facets into smaller bite size chunks. This does take quite a lot of self analysis and objectivity to 'see yourself' and why you do certain things and behave in certain ways. Actually it can be quite good therapy...

Remember back to your school days an opposition is 180', square 90', trine 120', and sextile 60' Now challenged aspects are conj (depending on the planets, cos some like each other and some really don't) squares and oppositions. Easy aspects are conj (if the planets get along) eg: Jupiter conj Moon, sextile and trine. Quincunx is inbetween (depending on the planets) causes health strains and unresolved or hard to balance planets/energies.

Now as there are 10planets in the sky and they ALL move so does your character 'progress' and in astrololgy these are called secondary progresions....
Your natal chart is like a photograph as unique as your fingerprint, BUT the 10planets in the sky have not stayed where they were in your natal chart, they have all moved. Predictive astrology is mostly maths where those 10planets are now and the mathematical aspects they make to your natal charts ie: 180'=opposition, 90'=square, 60'=sextile and 120'=trine.

So your personality, grows, develops, changes, matures as we get older, this is reflected by your sun sign changing from it's natal position to the next sign along, so does your Asc and MC sign change. In fact from sun to mars especially are important, as the outer planets really don't move much, using a 'day for a year' secondary progressions.